This was a very simple change and allowed us to remove the special
`json_opt_tok` macro.
Moved the callback out of `common/json.c` to `lightningd/json.c` because the new
callbacks are dependent on `struct command` etc.
(I already started on `json_tok_number`)
My plan is to:
1. upgrade json_tok_X one a time, maybe a PR for each one.
2. When done, rename macros (i.e, remove "_tal").
3. Remove all vestiges of the old callbacks
4. Add new callbacks so that we no longer need json_tok_tok!
(e.g., json_tok_label, json_tok_str, json_tok_msat)
Signed-off-by: Mark Beckwith <wythe@intrig.com>