Rethink about stream decoding

[dogtopus]

Some old version of MiniPB uses stream decoding exclusively. However due to how Protobuf handles repeated non-repeated fields (i.e. multiple fields with the same ID while the field is non-repeated) this was changed to pre-indexing the whole message and decode in one go.

We might still be able to bring stream decoding back by making the decode states read-write and return a snapshot of it when the decoding is done (by reaching an end-of-message marker either set by the user or naturally occurring).

This could also make implementing #11 easier and in a much cleaner way (instead of committing the states to a list/object, we just yield the state changes the first time we see them. It would be user's responsibility to only use the last one that pops up).