Cleanup for next major release: Port LocalRegisterCopy to `DataType`, `Read`, and `Write`.

[jrvanwhy]

#11 will introduce the DataType trait, which carries both the primitive type and bitfield type. Porting LocalRegisterCopy to DataType would allow us to unify its two generic arguments into one.

Also, LocalRegisterCopy's docs say:

/// This type does not implement the [`Readable`](crate::interfaces::Readable)
/// and [`Writeable`](crate::interfaces::Writeable) traits because it requires a
/// mutable reference to modify the contained value.

We should consider implementing Read, Write, and ReadWrite for &mut LocalRegisterCopy. I'm not sure whether we'd also want to implement Read for &LocalRegisterCopy as well -- we'd have to see what sort of name conflicts happen.