[mypyc] Support incremental compilation
This works by reworking IR generation to proceed a SCC at a time and
writing out caches of serialized IR information so that we can
generated code that calls into a module without compiling the module
in full. A mypy plugin is used to ensure cache validity by checking
that a hash of the metadata matches and that all of the generated source
is present and matches.
Closes mypyc/mypyc#682.
