In preparation for emptying the `internal_macros` module move the
`serde_string_impl` and `serde_struct_human_string_imp` macros to the
`serde_utils` module.
Rationale: `internal_macros` stuff can go over in the `internals` crate
now that we have one. The serde macros could go over there but we have a
`serde_utils` module that holds code for implementing serde traits,
these two macros are exactly that.
`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.
Tobin C. Harding