Renamed crypto_sign_blake2b_ctx back to crypto_sign_ctx

Renamed crypto_sign_blake2b_ctx back to crypto_sign_ctx

Also renamed crypto_check_blake2b_ctx back to crypto_check_ctx.

This serves two purposes: avoid breaking the API when users upgrade from
Monocypher 2.x, and keep the idea that Blake2b is the default hash (the
default settings are implied and need not be named).

Note that although old code is not broken, it will still have warnings.
Those are easily silenced by casting to (void*).