std.crypto.ecc: add support for the secp256k1 curve
Usage of the secp256k1 elliptic curve recently grew exponentially,
since this is the curve used by Bitcoin and other popular blockchains
such as Ethereum.
With this, Zig has support for all the widely deployed elliptic curves
today.
After P-256, here comes P-384, also known as secp384r1.
Like P-256, it is required for TLS, and is the current NIST recommendation for key exchange and signatures, for better or for worse.
Like P-256, all the finite field arithmetic has been computed and verified to be correct by fiat-crypto.
