Currently we have two macros used when creating array wrapper types,
one is in `internals` and the other in `bitcoin::internal_macros`. It
is not immediately obvious what is what and why there are two.
Improve the macros by:
- Move the inherent functions to `impl_array_newtype`
- Use `*_byte_array` for the names instead of `*_bytes` for functions
that return arrays
- Add `as_bytes` to return a slice
- Add `to_bytes` to return a vector
- Re-name the other macro to match what it now does
6ba7758b30