We currently duplicate the serde_round_trip macro in `units` and
`bitcoin`, this is unnecessary since it is a private test macro we can
just throw it in `internals`.
While we are at it lets improve the macro by testing a binary encoding
also, elect to use the `bincode` crate because we already have it in
our dependency graph.
Add `test-serde` feature to `internals` to feature gate the macro and
its usage (preventing the transient dependency on `bincode` and
`serde_json`).
7fa53440dc