Nothing major here:
1. size_t for lengths.
2. pass engine to checksum_verify, as caller wants ->len (avoid repeating 13/15 magic numbers).
3. Use x.member instesad of (&x)->member.
4. Return memcmp result directly instead of if.
5. Spacing removal, `;;` removal.
6. codexl is a bool `true`/`false` not 0/1 (it's the same, but clearer)
7. Make sanity_check assign *fail directly.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>