Change to use TryInto trait for signature array conversion

[franssl]

The conversion of signature &[u8] to &[u8;64] is currently implemented via a function that only works for this specific array length.
Once rust-lang/rust#61415 (const generics allow the TryInto trait to be implemented for any array size rather than just up to 32) is finished, this should be changed to use the TryInto trait (the key arrays are already using this).