Adding schnorrsig::KeyPair::from_secret_key convenience function

[dr-orlovsky]

No description provided.

[apoelstra]

ack bf9a276

[Tibo-lg]

@dr-orlovsky is there a reason why the function doesn't borrow the secret key instead of taking ownership of it?

(also post merge nit, the ::key:: prefix seems unnecessary)

[dr-orlovsky]

SecretKey's are copyable objects and impls for copyable objects should take self ownership, not reference, according to the Rust API guidelines

[Tibo-lg]

@dr-orlovsky Thanks for answering. Do you have a reference to that guideline? I looked up a bit but the best I found is this clippy rule which doesn't indicate anything for from https://rust-lang.github.io/rust-clippy/master/#wrong_self_convention

The reason I find it a bit confusing personally is because the from_secret_key for PublicKey takes it as a reference so it feels a bit inconsistent.

[elichai]

IMHO @dr-orlovsky is generally right that Copy types should be passed by value, because that allows the compiler to make better optimizations usually (unless it's a non-inlinable function and the Copy type is bigger than say ~64 bytes).

But here because the C function cares just about a pointer to the type and the rest of the functions already pass by reference I think it would've been better to be a reference