Deprecate this package

[roman-khimov]

Is your feature request related to a problem? Please describe.

I'm always frustrated when we're maintaining packages we no longer need. Go 1.24 provides RFC6979 ECDSA signatures out of the box.

Describe the solution you'd like

Check how it works, archive this package if it's OK and if it's not slower than this library.

Describe alternatives you've considered

Keep maintaining it.

Additional context

nspcc-dev/neo-go#3812

[roman-khimov]

Notice that crypto/ecdsa returns ASN.1 encoded signature, this can be inconvenient and require some adaptation (see #6 also).

[roman-khimov]

Implementation from core is ~30% slower:

BenchmarkECDSASign-16              60477             18239 ns/op            3779 B/op         52 allocs/op
BenchmarkECDSACore-16              45722             23930 ns/op            4184 B/op         63 allocs/op

It's supposed to be safer though because of constant time functions used instead of big.Int.

[roman-khimov]

Leaving this issue open but blocked, core implementation is still slower with Go 1.25 (see #15).

[roman-khimov]

Go 1.26:

BenchmarkECDSASign-16              78528             14787 ns/op            3659 B/op         48 allocs/op
BenchmarkECDSACore-16              58507             20295 ns/op            3912 B/op         55 allocs/op