perf: add kzg.UnsafeToBytes and kzg.UnsafeFromBytes methods

[gbotrel]

Description

TLDR; adds a method (no breaking changes) to kzg.SRS object that quickly dumps a binary representation without doing any checks, and optionally, enables to truncate the SRS at writing / reading time. UnsafeFromBytes is 6x time faster than UnsafeReadFrom

godoc:

// UnsafeToBytes returns the binary encoding of the entire SRS memory representation
// It is meant to be use to achieve fast serialization/deserialization and
// is not compatible with WriteTo / ReadFrom. It does not do any validation
// and doesn't encode points in a canonical form.
// @unstable: the format may change in the future
// If maxPkPoints is provided, the number of points in the ProvingKey will be limited to maxPkPoints
func (srs *SRS) UnsafeToBytes(maxPkPoints ...int) ([]byte, error) {

benchmark, 1 <<24 points bls12377

BenchmarkSerializeSRS/WriteTo-10                       1        1183526917 ns/op        2147545088 B/op      550 allocs/op
BenchmarkSerializeSRS/WriteRawTo-10                    1        1249923583 ns/op        4295028640 B/op      542 allocs/op
BenchmarkSerializeSRS/UnsafeToBytes-10                 4         287003656 ns/op        1610821252 B/op      522 allocs/op
BenchmarkDeserializeSRS/UnsafeReadFrom-10                      1        2677718292 ns/op        5922632568 B/op     1597 allocs/op
BenchmarkDeserializeSRS/UnsafeFromBytes-10                     3         419717778 ns/op        2147761242 B/op     1205 allocs/op

Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works
• I did not modify files generated from templates
• New and existing unit tests pass locally with my changes

[ivokub]

Small comments, but otherwise looks great.