Support for empty statement

[riz0id]

This PR adds support for parsing and discarding empty statements in enumerations, messages, and services. As well, this PR removes the "empty" constructors in DotProtoServicePart and DotProtoMessagePart as they're no longer used. This PR resolves issue #257.

[j6carey]

In addition to the specifics: are you planning to keep DPSEmpty? Having an explicit representation of an empty top-level statement shows a different style than the approach you are taking with this PR in service, message, and enum definitions.

[j6carey]

Thanks for the improvement. I still wonder if we should do something about DPSEmpty being handled differently, but I don't want to block this PR for that.

[riz0id]

DPSEmpty

Thanks for the improvement. I still wonder if we should do something about DPSEmpty being handled differently, but I don't want to block this PR for that.

Typically the top-level needs to be handled differently. I think leaving it for now is fine, but if it can be removed then I think we should remove it in the future.