modsys: Land gc() function here. Unfinished.

This is RFC for providing port-independent location for gc() call. CPython has a special module "gc" to deal with gc, but apparently, it's overkill to introduce that for ~1 function (such module can be made on Python level). Sys seams like a natural choice, even if such name there is not compliant with CPython. "micropython" module would seem other choice, but that IMHO is hints its implementation-specific, while generally concept of GC is not.

This is just initial RFC, which, if approved, should be refactored further. A next question then will be - should "gc" be removed from "pyb" module with this change.