Implement rudimentary "reserved" field support

[thirtythreeforty]

This implementation does not validate that the field names and/or numbers are not reused; it simply ignores them. This allows parsing files that contain the reserved keyword.

[tburmeister]

Thanks! Looks pretty good but...

1. We should add a test
2. I think this only partially works, based on the example in the language guide:

message Foo {
  reserved 2, 15, 9 to 11;
  reserved "foo", "bar";
}

As is, something like reserved 2, 15 will work, but this will fail for 9 to 11 or "foo", "bar".