Move trait bounds on `wire::Type` from use to the trait itself

Move trait bounds on `wire::Type` from use to the trait itself

`wire::Type` is only (publicly) used as the `CustomMessage`
associated type in `CustomMessageReader`, where it has additional
trait bounds on `Debug` and `Writeable`. The documentation for
`Type` even mentions that you need to implement `Writeable` because
this is the one place it is used.

To make this more clear, we move the type bounds onto the trait
itself and not on the associated type.

This is also the only practical way to build C bindings for `Type`
as we cannot have a concrete, single, `Type` struct in C which only
optionally implements various subtraits, at least not without
runtime checking of the type bounds.