Add encode_to_slice

[Luro02]

This is the counterpart to decode_to_slice.

Things to consider

Should the function accept a trait as input?

This would be the most generic approach:

pub fn encode_to_slice<T>(input: &T, output: &mut [u8]) -> Result<(), FromHexError>
where
    T: ::core::iter::IntoIter<Item = u8>,

or the same signature, that encode has?

pub fn encode_to_slice<T: AsRef<[u8]>>(input: &T, output: &mut [u8]) -> Result<(), FromHexError>

I personally would prefer to keep the signature, that it has now.

pub fn encode_to_slice(input: &[u8], output: &mut [u8]) -> Result<(), FromHexError>;

[KokaKiwi]

Personally i would prefer having a signature similar to decode_to_slice (and thus encode and decode as well), as in accepting a type implementing AsRef<[u8]>, for consistency.