Use operator precedence to print C code

[JasonGross]

I'm attempting to make the printing code more uniform and factor out common patterns a bit more, in part to decrease the likelihood of transcription errors, but I'm afraid I might be making this code harder to read and understand. I'd love feedback on this @andres-erbsen @jedisct1 (and anyone else interested in looking at the backend code). The relevant file to look at is src/Stringification/C.v

This is also in part to allow implementing @FiloSottile's request in #949 to not have so many useless parentheses.

[jedisct1]

This is certainly making the output look nicer.

However I'm not convinced that this is necessary. Extra parenthesis don't make the compiled code run any slower, and this is automatically generated code, that will not be manually modified.

The Coq code is far more complex to understand, which may hurt addition of new languages.

[lorandam]

I think your doing great