feat: Refactor ParseDN function to fix resource usage and invalid parsings (fixes #487)

[cpuschma]

This PR reworks the ParseDN function, which has been very resource consuming for reading one byte at a time and couldn't handle certain characters. This implementation is based upon inteons PR for cert-manager, which wasn't fully compatible with the RFC 4514.

This PR also:

• Moves an unexported variable to a function scoped constant to free up the name hex
• Renames the imported package enchex to it's original name hex for better readabilty
• Improves the readability of ParseDN test failures

If this resolves the reported problems, we can revert #466.

[cpuschma]

Additionally, results of some benchmarking benchmark:

Summary

goos: linux
goarch: amd64
pkg: github.com/go-ldap/ldap
cpu: AMD Ryzen 5 5600X 6-Core Processor             
                │   old.txt    │               new.txt               │
                │    sec/op    │   sec/op     vs base                │
ParseSubject-12   2392.0n ± 1%   754.2n ± 1%  -68.47% (p=0.000 n=15)

                │   old.txt   │              new.txt               │
                │    B/op     │    B/op     vs base                │
ParseSubject-12   1865.0 ± 0%   408.0 ± 0%  -78.12% (p=0.000 n=15)

                │  old.txt   │              new.txt               │
                │ allocs/op  │ allocs/op   vs base                │
ParseSubject-12   58.00 ± 0%   10.00 ± 0%  -82.76% (p=0.000 n=15)

Old implementation:

BenchmarkParseSubject-12    	  673922	      2258 ns/op	    1865 B/op	      58 allocs/op
BenchmarkParseSubject-12    	  519794	      2403 ns/op	    1865 B/op	      58 allocs/op
BenchmarkParseSubject-12    	  458138	      2411 ns/op	    1865 B/op	      58 allocs/op
BenchmarkParseSubject-12    	  426906	      2440 ns/op	    1865 B/op	      58 allocs/op
BenchmarkParseSubject-12    	  502710	      2369 ns/op	    1865 B/op	      58 allocs/op

New implementation::

BenchmarkParseSubject-12    	 1606474	       773.1 ns/op	     408 B/op	      10 allocs/op
BenchmarkParseSubject-12    	 1554079	       768.1 ns/op	     408 B/op	      10 allocs/op
BenchmarkParseSubject-12    	 1594885	       760.0 ns/op	     408 B/op	      10 allocs/op
BenchmarkParseSubject-12    	 1564299	       754.2 ns/op	     408 B/op	      10 allocs/op
BenchmarkParseSubject-12    	 1547341	       755.9 ns/op	     408 B/op	      10 allocs/op

[johnweldon]

LGTM, except for the comment typo

[inteon]

@cpuschma @johnweldon
Great to see you are improving this function!
There is one small issue with this approach that I also figured out too late: asn1.Unmarshal( only supports DER parsing, while the github.com/go-asn1-ber/asn1-ber library supports BER parsing too.
This means that parseDN deviates from the RFC, possibly resulting in unexpected limitations for the user.

[cpuschma]

I'll look into this and also extend the test cases. We didn't catch on that either. Thank you for pointing this out, @inteon!

[inteon]

I also have some extra testcases that might be useful. Will create a PR soon.