Use reusable library for field access code

[hannobraun]

This crate contains custom field access code. Since this code isn't in any way specific to this crate, I think it would be better to move it out.

I looked for libraries before implementing the code. The only thing that came close to what I needed was bitfield, but that doesn't support byte slices as the type of the bit field, which is what I needed.

I think this issue could be solved in either of these 3 ways:

1. Find another library that fulfills our needs and use that.
2. Extend the bitfield crate and use that.
3. Clean up the custom code and move it to its own library.