Move the `opcodes` module to the new `primitives` crate. This is pretty
straight forward, some things to note:
- Are we ok with the public wildcard re-export from `blockdata`? I think
so because the whole `blockdata` module should, IMO, be deleted after
everything in it is moved to `primitives`.
- `decode_pushnum` becomes public.
Includes addition of a `patch` section for `primitives` in the
`bitcoin/embedded` crate.
9a586987d1