freebox,space,monopam,meross: let open Json.Codec in cleanup
Per-let `let open Json.Codec in` inside each codec body. freebox
needed a few API-helper sites to keep `Json.Codec.list` and
`Json.Codec.null` qualified (they sit outside any codec body, so
opening locally would be wasteful), and `(l : l2ident)` annotation
on freebox/hosts l.id to disambiguate from mem_map.id.
space/config has similar Json.Codec.list at the annotation/encoding
sites that wrap codecs into HTTP requests.
monopam (changes/changes_aggregated/changes_daily) and
meross/device have no field clashes; opens applied straight through.