perf: unsafe.WriteSlice (#501)

[gbotrel]

#501 can't be rebased -- recreating the PR on top of master.

• refactor: expose pedersen basis slices
• feat: more elegant solution to mem dump using unsafe and generics

Motivation; enable other gnark objects (like groth16.ProvingKey) to be written and read very fast (though not portable). Limit code duplication between curves, fields, proof systems, etc.

• Adds utils/unsafe with WriteSlice[S ~[]E, E any](w io.Writer, s S) error ... ;
• kzg.UnsafeToBytes(...) []byte is now WriteDump(w io.Writer) error

🔥 Benchmarks for kzg SRS dump 🔥 --> (I mean using unsafe is a cheat, we just benchmark the non-unsafe method + real IO)

benchmark                                      old ns/op      new ns/op      delta
BenchmarkSerializeSRS/WriteTo-10               1227334792     1200767042     -2.16%
BenchmarkSerializeSRS/WriteRawTo-10            1291239625     1275224792     -1.24%
BenchmarkSerializeSRS/UnsafeToBytes-10         288414938      49002102       -83.01%
BenchmarkDeserializeSRS/UnsafeReadFrom-10      1589721583     1607936042     +1.15%
BenchmarkDeserializeSRS/UnsafeFromBytes-10     283540219      81233306       -71.35%

benchmark                                      old allocs     new allocs     delta
BenchmarkSerializeSRS/WriteTo-10               540            540            +0.00%
BenchmarkSerializeSRS/WriteRawTo-10            542            541            -0.18%
BenchmarkSerializeSRS/UnsafeToBytes-10         522            512            -1.92%
BenchmarkDeserializeSRS/UnsafeReadFrom-10      1046           1047           +0.10%
BenchmarkDeserializeSRS/UnsafeFromBytes-10     1029           1031           +0.19%

benchmark                                      old bytes      new bytes      delta
BenchmarkSerializeSRS/WriteTo-10               2147544696     2147544696     +0.00%
BenchmarkSerializeSRS/WriteRawTo-10            4295028640     4295028632     -0.00%
BenchmarkSerializeSRS/UnsafeToBytes-10         1610821246     67176202       -95.83%
BenchmarkDeserializeSRS/UnsafeReadFrom-10      1627598752     1627598848     +0.00%
BenchmarkDeserializeSRS/UnsafeFromBytes-10     1610820720     1610820744     +0.00%