Build profiles

1. "pkg.mypy.multifile": for the self-compilation of mypy using mypyc, forces
   the use of "multi_file" option. Useful for CI, where build time limits might
   otherwise prevent successful compilation.
   "This can reduce compile time and memory requirements at the likely
   cost of runtime performance of compiled code."

2. "pkg.mypy.o1": for the self-compilation of mypy using mypyc, forces
   the optimization level ("MYPYC_OPT_LEVEL") to be "1", which correspond to
   compilation with "-O1". Only set during package building, not set while
   running the tests. Useful for CI, where build time limits might otherwise
   prevent successful compilation.
