Create a Nibbles newtype

[rkuris]

We're very sloppy about what is a nibble and what is an array of constructed u8 arrays. Consider making this more explicit within the codebase by wrapping nibbles into a newtype.

Originally posted by @richardpringle in #435 (comment)

          I think this would be great if we had `Item = Nibble` where `#[repr(transparent)] struct Nibble(u8)` (where you can only create the nibble if it's within the correct bounds).

Instead, I think something like this would make the call more explicit

/// CAUTION: only use with nibble iterators  
pub(crate) fn nibbles_into_bytes<Iter: Iterator<Item = u8>>(iter: Iter) -> Vec<u8> {
    let mut data = Vec::with_capacity(self.size_hint().0 / 2);

    while let (Some(hi), Some(lo)) = (self.next(), self.next()) {
        data.push((hi << 4) + lo);
    }

    data
}

[rkuris]

This is partially solved by new design, but could use a 👀 to see if we can do better.