(Re)add enum support

[arnetheduck]

As part of the cleanup in #36, enum suppot was removed to simplify the implementation - support for dealing with protobuf enums should eventually be readded, though it must conform to:

https://developers.google.com/protocol-buffers/docs/proto3#enum and https://developers.google.com/protocol-buffers/docs/proto2#enum respectively - in particular, defaults and unknown values need special handling compared to Nim enum.

In the meantime, enum fields can be encoded as int32/pint or uint32/pint with constants for values