Manage complexity of the C frontend

[bmourad01]

As discussed in #167, the Patch_c module is quite extensive already. The main meat of it is a large chain of mutually recursive functions required for doing the bulk of the translation from FrontC to PatchC. It would be worth considering how to manage this complexity better. In particular, @ivg suggested a scheme where open recursion could be used, thereby allowing each related group of functions to be split into their own units/submodules.