This is an omnibus commit of the last few evenings' changes. Primarily it was to start laying the groundwork for constructors, but it includes other fixes and closes a few issues.

Details:

- Begin infrastructure preparation for constructors

- Started creating navigation APIs to replace the low-level graph node chasing; this makes cppfront's own code cleaner and the tree easier to change if needed, but it's also a step toward a reflection API

- Extended `main:(args)` to require the name "args" for the single-parameter `main`, and to support `args.argc` and `args.argv`, further on hsutter#262 (see comment thread)

- Changed default return type for unnamed functions to be `-> void`, same as named functions, closes hsutter#257

- Disallow single-expression function body to be just `return`, closes hsutter#267

- Make `make_args` inline, closes hsutter#268

- Bug fix: emit prolog also for single-expression function body. Specifically, this enables main:(args)=expression; to work correctly. Generally, this also corrects the code gen for examples that were trying (but failing) to inject prologs in single-expression functions... in the regression tests this corrected the code gen for `pure2-forward-return.cpp2` which was missing the contract check before.