proto: Support map fields

[euroelessar]

There is a TODO in the codebase, so I assume the support is desired.
https://github.com/google/starlark-go/blob/master/lib/proto/proto.go#L84

There are two primary questions in regard of the possible implementation:

1. Ensuring deterministic ordering is tricky, the only feasible way is to sort all keys before the iteration (meaning lexicographical order as in opposite to the insertion one in regular dicts).
2. Are there any tests for the package? Where should they be put?

[tazjin]

I'm very interested in sorting this out at the moment, at least for message initialisation. @adonovan, would you accept a PR for this? (CLA is on file, and I'm a Xoogler).

[tazjin]

Turned out to be fairly easy, map construction is here: #491

It seems like there isn't great test coverage of the lib/proto functionality at the moment. Didn't touch that too much. This code exists in a private fork, too, and has been tested against some fairly complex protos with map fields there.