Go 1.26 support

[tarakby]

closes #40

• ECDSA on P-256 is based on Go crypto standard lib implements (no changes)
• ECDSA on SECp256k1 is based on go-etehereum/crypto/secp256k1
  • this requires cgo, CGO_ENABLED=1, in the past version SECp256k1 did not use cgo
  • signatures generation is now based on RFC6979. The signatures are therefore deterministic. Moreover they are always normalized with a lower S.
  • verification still accepts both low S and high S versions of the valid signature. Although the underlying go-ethereum verification enforces non-malleability (low S only), signatures in this package are still malleable (no change compared to prior versions).

This PR also removes the non-cgo build mode, which is used to build the library without cgo (CGO_ENABLED=0) while disabling BLS. Now that ECDSA secp256k1 also uses cgo, the non-cgo mode has very limited utility and is discontinued.

[turbolent]

I reviewed the code from a Go-perspective, but I don't have the expertise to review the crypto changes

[turbolent]

Could you please improve the PR title?