introduction of prefixed map

Prefixed map is a new storage available in decl_storage, it differ from
map as it stores it keys at `twox_256(prefix)++map_hasher(key)` instead
of `map_hasher(prefix ++ key)`. Thus it allows to remove all key of the
map using `sr-io::kill_prefix`, but the final goal is to also allow to
iter on the map once sr-io make such call available.