Preserve unknown proto2 fields and enums

[judah]

We should preserve unknown fields and enums in proto2 messages.

From the docs: https://developers.google.com/protocol-buffers/docs/proto

Similarly, messages created by your new code can be parsed by your old code: old binaries simply ignore the new field when parsing. However, the unknown fields are not discarded, and if the message is later serialized, the unknown fields are serialized along with it – so if the message is passed on to new code, the new fields are still available.

Also, enums with unknown values should be treated like unknown fields and preserved for reserialization. (Note that this behavior is proto2-only; #28 describes the desired behavior for proto3.)

[judah]

Update: as described in google/protobuf#272, the specification is changing to preserve unknown fields in proto3 as well as proto2. See also: https://docs.google.com/document/d/1KMRX-G91Aa-Y2FkEaHeeviLRRNblgIahbsk4wA14gRk/edit#heading=h.w8dtggryroj4

When we implement this behavior, we should follow the new spec.

[judah]

This has been mostly done for unknown fields (but not enums), in #129.

There are a couple remaining edge cases:

• Printing unknown groups is more unstructured than it could be
• The discardUnknownFields function doesn't act recursively (unlike other languages)