Abstract out iterating over a Message's Fields. (#120)
Abstract out iterating over a Message's Fields. (#120)

Add methods `Message::all_fields` and `Message::all_fields_mut` that return
iterators over all a Message's fields, including those in `oneof` blocks. Use
these new methods to abstract out such iterations in the existing code.
tafia/quick-protobuf,tafia/quick-protobuf