Represent signature subpackets in an ordered map

We only want one of each signature subpacket per signature, and we need to preserve ordering when parsing in order to compute correct hashes.

An ordered map of some sort would make this easier, and would get rid of the List.filter/List.for_all/List.exists mess. Typed functions to retrieve subpackets of a certain kind would also get rid of a lot of warnings.

TODO find some module that exposes a usable Map module, or write one.