`impl_array_newtype` is an internal macro, move it to a new, ever so
meaningfully named, `macros` module.
Use `#[macro_export]`, no other changes to the macro.
We are attempting to flatten the `util` module; move the `bip32` module
to the crate root out of `util`.
Currently `src/util/` is ignored by the formatter, this patch does not
do formatting, will be done later.