Bart/prelim types

[Horusiath]

This PR adds two things:

1. Prelim trait which can be used to support preliminary types, useful when you want to ie. create already populated YMap or YArray and nest it into another Y-collection type.
2. Value enum which is used to return values stored inside of YMap, YArray etc. Previously we used Any for that, but the problem is that Any only represent primitive JSON-like values, and we had no way to return YMap nested inside of another YMap etc. For this reason Value enum allows us to return both primitive types as well as complex ones (maps, arrays, xml elements and text), and operate on them. Value can also be converted into Any if transaction reference was provided.