[RFC] Codecs for items in order of their appearance

[chshersh]

After using tomland on non-trivial use-cases, @vrom911 and I realized, that sometimes it's desirable to parse TOML items in order of their appearance. In tomland we have an intermediate data type, that represents such TOML, we only to write codecs for it. But I want to avoid is to duplicate all codecs for this new representation. So we may want to introduce a module like Toml.Codec.Sequential with some bridge codecs.