Streamline hashing interface of Tip5

[jan-ferdinand]

Tip5 has the following functions for hashing:

pub fn hash_10(input: &[BFieldElement; 10]) -> [BFieldElement; Digest::LEN] { … }
pub fn hash_pair(left: Digest, right: Digest) -> Digest { … }
pub fn hash<T: BFieldCodec>(value: &T) -> Digest { … }
pub fn hash_varlen(input: &[BFieldElement]) -> Digest { … }

This feels somewhat excessive. Try to streamline the interface, or document the differences and each function's necessity.