Add EncodeInto to types.Point for allocation-free encoding

[jorgecuesta]

Description

This PR introduces a new EncodeInto(dst []byte) int method on types.Point, implemented for both secp256k1 and ed25519 points.

The goal is to allow callers (like ring-go) to reuse buffers when serializing points, instead of always allocating a fresh slice. This is particularly helpful inside signature verification loops where point encoding is on the hot path.

Key changes:

• Added EncodeInto to types.Point interface.
• Implemented EncodeInto for secp256k1 and ed25519 points.
• Added benchmarks to compare Encode vs EncodeInto and demonstrate that:
  • Neutral on secp256k1 (no regressions).
  • Fewer allocations on ed25519 (allocation-free path).
• Added simple tests to ensure EncodeInto produces the same bytes as Encode.

Benchmarks

Ran on AMD Ryzen 9 5950X, Go 1.23:

BenchmarkChallenge_Secp256k1/with_EncodeInto-32       65546   17426 ns/op   1483 B/op   12 allocs/op
BenchmarkChallenge_Secp256k1/with_Encode-32           67558   17454 ns/op   1483 B/op   12 allocs/op

BenchmarkChallenge_Ed25519/with_EncodeInto-32        171180    6961 ns/op   1064 B/op    6 allocs/op
BenchmarkChallenge_Ed25519/with_Encode-32            167292    7025 ns/op   1128 B/op    8 allocs/op

BenchmarkEncodeInto_vs_Encode_Secp256k1/EncodeInto-32 150338   8136 ns/op     48 B/op    1 allocs/op
BenchmarkEncodeInto_vs_Encode_Secp256k1/Encode-32     147151   8121 ns/op     48 B/op    1 allocs/op

BenchmarkEncodeInto_vs_Encode_Ed25519/EncodeInto-32  402760    2935 ns/op      0 B/op    0 allocs/op
BenchmarkEncodeInto_vs_Encode_Ed25519/Encode-32      401104    2988 ns/op     32 B/op    1 allocs/op

Summary

• ✅ Keeps backward compatibility (Encode unchanged).
• ✅ Provides an allocation-free path for projects that want to reuse buffers.
• ✅ Neutral on secp256k1, positive on ed25519.
• ✅ Includes tests and benchmarks to validate behavior.