This avoids any confusion between primitive and string ids, and in
particular stops an issue with commando once it starts chaining ids,
that weird ids can be double-escaped and commando will not recognize
the response, leaving the client hanging. It's the client's fault for
using a weird id, but it's still rude (and triggered by our tests!).
It also makes substituting the id in passthrough simpler, FTW.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>