we're attempting to stay as close to mainline chibicc as possible

at a high level, the major changes made are:
- strip out most of proprocess.c, as we're OK with using GCC's preprocessor
  (still need preprocess.c, as it handles parsing of literals)
- replace codegen.c to output C instead of assembly

to support these, ended up making some changes to the rest of the files. these
include:
  - in type.c:usual_arith_conv,
    - don't cast if they're already the right type
    - for pointer arithmetic, leave it as pointer + long
  - in parse.c:is_const_expr, also allow modulos to be const