Things to finish before moving onto compiler optimizations, C extensions, other nice features

- regalloc: fix phi-eliminating moves
- frontend: try to repr vars as ssa temps until they have their address taken or are mutated (to reduce no. of mem instrs)??
- ir: implement mem2reg
- frontend: finish C impl: initializers, preprocessor (#include, fn-like macros, etc), forward declarations, for, switch, break-continue, goto

at some point add another backend like arm64 to make sure the non target specific stuff is generic enough..