Its not immediately obvious why we nest the whole `io` code in an `io`
submodule within `lib.rs`. As far as I can tell we can inline it and
re-export from `rust-bitcoin` same as we do for our other dependencies.
This change would effect other users of the crate but since the `io`
crate is unreleased this effects no-one except us.
In preparation for inlining the `io` molule, move the public macros to a
private `macros` module.
Includes removal of the public re-export of `std` as `_std` - flaggin
this because I do not understand why it is here in the first place, we
can use `std::io::Write` in code that is feature gated on "std".