Use generics to generalize int32/int64/float32/float64 to number encoder/decoder/store

[akrennmair]

This PR introduces Go 1.18 generics to generalize the separate int32/int64/float32/float64 implementation into a single number type with small specializations. This deduplicates a lot of similar code across these types.

Since this required a switch to Go 1.18, I also replaced interface{} with the new any type. I also migrated the existing fuzz tests for gofuzz with Go 1.18 fuzz tests.

And finally, I removed the vendor subdirectory as it caused issues and doesn't really make sense for a library.

[akrennmair]

Looks like golangci-lint isn't quite mature enough to lint this code. Converting to draft for now...

[codecov-commenter]

Codecov Report

Merging #83 (d41620e) into master (e6b70a8) will increase coverage by 0.53%.
The diff coverage is 92.47%.

@@            Coverage Diff             @@
##           master      #83      +/-   ##
==========================================
+ Coverage   86.73%   87.27%   +0.53%     
==========================================
  Files          48       46       -2     
  Lines        9777     9444     -333     
==========================================
- Hits         8480     8242     -238     
+ Misses        939      884      -55     
+ Partials      358      318      -40     

Impacted Files	Coverage Δ
cmd/parquet-tool/cmds/readfile.go	0.00% <0.00%> (ø)
floor/interfaces/unmarshaller.go	75.93% <75.00%> (ø)
helpers.go	70.73% <75.00%> (+0.49%)	⬆️
deltabp_decoder.go	65.85% <76.19%> (+6.94%)	⬆️
type_dict.go	73.25% <77.77%> (ø)
deltabp_encoder.go	64.17% <78.57%> (-8.67%)	⬇️
floor/interfaces/marshaller.go	88.57% <85.71%> (ø)
schema.go	74.58% <85.71%> (ø)
type_int96.go	78.18% <87.50%> (ø)
chunk_writer.go	84.09% <90.00%> (-0.79%)	⬇️
... and 22 more

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