index: Add missing integer indexers

[joamaki]

Add the missing integer indexers and simplify all by using Append*.

[github-actions]

$ make test
go: downloading github.com/mitchellh/mapstructure v1.5.0
go: downloading golang.org/x/term v0.16.0
go: downloading github.com/fsnotify/fsnotify v1.7.0
go: downloading github.com/sagikazarmark/slog-shim v0.1.0
go: downloading github.com/spf13/afero v1.11.0
go: downloading github.com/spf13/cast v1.6.0
go: downloading golang.org/x/sys v0.17.0
go: downloading github.com/subosito/gotenv v1.6.0
go: downloading github.com/hashicorp/hcl v1.0.0
go: downloading gopkg.in/ini.v1 v1.67.0
go: downloading github.com/magiconair/properties v1.8.7
go: downloading github.com/pelletier/go-toml/v2 v2.1.0
go: downloading golang.org/x/text v0.14.0
	github.com/cilium/statedb/reconciler/benchmark		coverage: 0.0% of statements
	github.com/cilium/statedb/reconciler/example		coverage: 0.0% of statements
ok  	github.com/cilium/statedb	4.811s	coverage: 87.5% of statements
ok  	github.com/cilium/statedb/index	0.004s	coverage: 31.1% of statements
ok  	github.com/cilium/statedb/internal	0.010s	coverage: 93.3% of statements
ok  	github.com/cilium/statedb/part	1.206s	coverage: 87.8% of statements
ok  	github.com/cilium/statedb/reconciler	0.610s	coverage: 89.9% of statements
-----
$ make bench
go test ./... -bench . -benchmem -test.run xxx
goos: linux
goarch: amd64
pkg: github.com/cilium/statedb
cpu: AMD EPYC 7763 64-Core Processor                
BenchmarkDB_WriteTxn_1-4                    	  530596	      2312 ns/op	    432555 objects/sec	    2600 B/op	      31 allocs/op
BenchmarkDB_WriteTxn_10-4                   	 1302624	       874.8 ns/op	   1143086 objects/sec	     681 B/op	      10 allocs/op
BenchmarkDB_WriteTxn_100-4                  	 1762243	       684.4 ns/op	   1461208 objects/sec	     575 B/op	       7 allocs/op
BenchmarkDB_WriteTxn_1000-4                 	 1651500	       725.3 ns/op	   1378780 objects/sec	     543 B/op	       7 allocs/op
BenchmarkDB_WriteTxn_10000-4                	 1350386	       896.6 ns/op	   1115283 objects/sec	     596 B/op	       7 allocs/op
BenchmarkDB_WriteTxn_100_SecondaryIndex-4   	  895143	      1333 ns/op	    750063 objects/sec	     970 B/op	      15 allocs/op
BenchmarkDB_RandomInsert-4                  	    2493	    466022 ns/op	   2145820 objects/sec	  423236 B/op	    7153 allocs/op
BenchmarkDB_RandomReplace-4                 	     726	   1652411 ns/op	    605176 objects/sec	 1155063 B/op	   16506 allocs/op
BenchmarkDB_SequentialInsert-4              	    1712	    730805 ns/op	   1368354 objects/sec	  543181 B/op	    7221 allocs/op
BenchmarkDB_Changes_Baseline-4              	    1306	    897891 ns/op	   1113722 objects/sec	  582970 B/op	   10301 allocs/op
BenchmarkDB_Changes-4                       	     716	   1649952 ns/op	    606078 objects/sec	 1041041 B/op	   14646 allocs/op
BenchmarkDB_RandomLookup-4                  	   24739	     49263 ns/op	  20299384 objects/sec	     144 B/op	       1 allocs/op
BenchmarkDB_SequentialLookup-4              	   30738	     39439 ns/op	  25355689 objects/sec	       0 B/op	       0 allocs/op
BenchmarkDB_FullIteration_All-4             	     865	   1391827 ns/op	  71848160 objects/sec	     360 B/op	       7 allocs/op
BenchmarkDB_FullIteration_Get-4             	     243	   4861142 ns/op	  20571335 objects/sec	       0 B/op	       0 allocs/op
BenchmarkDB_PropagationDelay-4              	  522382	      2066 ns/op	        18.00 50th_µs	        21.00 90th_µs	        82.00 99th_µs	    1439 B/op	      23 allocs/op
PASS
ok  	github.com/cilium/statedb	25.434s
PASS
ok  	github.com/cilium/statedb/index	0.004s
PASS
ok  	github.com/cilium/statedb/internal	0.003s
goos: linux
goarch: amd64
pkg: github.com/cilium/statedb/part
cpu: AMD EPYC 7763 64-Core Processor                
Benchmark_Insert_RootOnlyWatch-4    	    9264	    122536 ns/op	   8160870 objects/sec	  119104 B/op	    2067 allocs/op
Benchmark_Insert-4                  	    6978	    170730 ns/op	   5857216 objects/sec	  219614 B/op	    3105 allocs/op
Benchmark_Replace-4                 	47719894	        25.14 ns/op	  39773765 objects/sec	       0 B/op	       0 allocs/op
Benchmark_Replace_RootOnlyWatch-4   	46585112	        25.11 ns/op	  39827158 objects/sec	       0 B/op	       0 allocs/op
Benchmark_txn_1-4                   	 3812241	       325.6 ns/op	   3070916 objects/sec	     336 B/op	       6 allocs/op
Benchmark_txn_10-4                  	 9003589	       132.2 ns/op	   7564746 objects/sec	     142 B/op	       2 allocs/op
Benchmark_txn_100-4                 	 8833093	       138.5 ns/op	   7222383 objects/sec	     211 B/op	       2 allocs/op
Benchmark_txn_1000-4                	 8644692	       138.2 ns/op	   7235813 objects/sec	     211 B/op	       2 allocs/op
Benchmark_txn_10000-4               	 5565459	       216.3 ns/op	   4623981 objects/sec	     224 B/op	       2 allocs/op
Benchmark_txn_100000-4              	 4160146	       265.2 ns/op	   3771123 objects/sec	     335 B/op	       2 allocs/op
Benchmark_txn_delete_1-4            	 3733228	       319.4 ns/op	   3130981 objects/sec	     840 B/op	       6 allocs/op
Benchmark_txn_delete_10-4           	10431166	       114.7 ns/op	   8721590 objects/sec	     129 B/op	       1 allocs/op
Benchmark_txn_delete_100-4          	13079502	        91.48 ns/op	  10931492 objects/sec	      61 B/op	       1 allocs/op
Benchmark_txn_delete_1000-4         	14524563	        82.45 ns/op	  12127980 objects/sec	      26 B/op	       1 allocs/op
Benchmark_txn_delete_10000-4        	11617999	       101.8 ns/op	   9822058 objects/sec	      25 B/op	       1 allocs/op
Benchmark_txn_delete_100000-4       	10033552	       110.4 ns/op	   9061023 objects/sec	      26 B/op	       1 allocs/op
Benchmark_Get-4                     	   50340	     24813 ns/op	  40301703 objects/sec	       0 B/op	       0 allocs/op
Benchmark_Iterate-4                 	  167383	      7342 ns/op	 136210695 objects/sec	      80 B/op	       3 allocs/op
Benchmark_Hashmap_Insert-4          	   15630	     76905 ns/op	  13003102 objects/sec	   86552 B/op	      64 allocs/op
Benchmark_Hashmap_Get_Uint64-4      	  153154	      7764 ns/op	 128802054 objects/sec	       0 B/op	       0 allocs/op
Benchmark_Hashmap_Get_Bytes-4       	  154455	      7804 ns/op	 128147669 objects/sec	       0 B/op	       0 allocs/op
PASS
ok  	github.com/cilium/statedb/part	29.538s
PASS
ok  	github.com/cilium/statedb/reconciler	0.005s
?   	github.com/cilium/statedb/reconciler/benchmark	[no test files]
?   	github.com/cilium/statedb/reconciler/example	[no test files]
go run ./reconciler/benchmark -quiet
100000 objects reconciled in 0.26 seconds (batch size 1000)
Throughput 384147.45 objects per second
Allocated 600533 objects, 40870kB bytes, 49896kB bytes still in use

[bimmlerd]

lgtm ;)