Make k256 default precompile

[rakita]

This bench shows that k256 is faster than secp256k1 (C version): https://github.com/tarcieri/rust-secp256k1-ecdsa-bench. We should default to using it.

[aguzmant103]

@rakita I might be missing something but seems rust-secp256k1 is fastest right? Then k256 then parity's version libsec256k1. So we'd want to default the first one?
Also happy to take a stab at it! 🙋‍♂️

[Pana]

k256 and libsec256k1 are implemented in pure Rust

[FredCoen]

once #1915 is merged should it default to rust-secp256k1 and not k256?

[rakita]

@aguzmant103 you are right I made mistake here, from graph [rust-secp256k1](https://github.com/rust-bitcoin/rust-secp256k1) is fastest and should stay as default.

Will close this issue as @FredCoen is adding libsec256k1 support in #1915.