Add comments to C syntax tree as statements.

Comments are a constructor for the the 'Stm' data type. They will prettyprint, but are not implemented in the parser or quasiquoter.

I implemented this so we can emit comments along with generated C code, to make it easier to debug the generated code. I'm open to implementing comments in the parser as well, but I'm not quite clever enough to figure out how, and it isn't useful for my application at the moment. I'm happy to discuss a strategy and work on it more if there is interest.