Add nibbles_into_bytes for an arbitrary iterator

[rkuris]

This method is useful for converting nibbles back into their corresponding bytes.

Added some simple tests, both in the success case and the failure case.

[rkuris]

This PR came to be from the review of #430

After applying this diff, the code in that PR key_from_nibbles_iter moves here, so the one caller can just do:

use nibbles::IntoBytes;
/// ... then in `find_next_result`:
let key = nibble_iter_from_parents(parents).chain(node_path_iter).nibbles_into_bytes();

[richardpringle]

Could we identify where this would be used outside of stream.rs before we merge? If the use case only exists in one module, I think the abstraction should live there until we find more use cases.

[richardpringle]

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]

Created #440 for this larger change

[rkuris]

Could we identify where this would be used outside of stream.rs before we merge? If the use case only exists in one module, I think the abstraction should live there until we find more use cases.

I couldn't find any very quickly so I moved this to stream.rs and make it less visible elsewhere. Lets hope that when someone does find the other use cases that they also find this code.