Make ucl_object_find_key[l] support finding an array index, as well as a
key

Make ucl_object_insert_key_common (and by
extension: ucl_object_replace_key, ucl_object_insert_key_merged, etc)
access an array index in place of a key if the object is an array
Remove some debugging
Remove more debugging
Implement ucl_object_mix_key, to combine 'merge' and 'replace' modes,
to overwrite keys that conflict, rather than creating arrays
Implement ucl_array_merge to merge two arrays