Generate Haddock from C-style comments

[FintanH]

I was mulling over protobuf documentation. I thought it would be nice if the C-style comments could turn into Haddock documentation for messages and possibly their fields. I dunno how possible this is though.

[judah]

This ought to be possible in principle. The descriptor proto message that we get from protoc has the comments attached to individual entities via SourceCodeInfo:
https://github.com/google/protobuf/blob/b40cb4bc8bd233330cba2ab09d6e3016b004dce0/src/google/protobuf/descriptor.proto#L728
Seems like we could parse them out into Haddocks, possibly escaping any special Haddock characters.