This is meant to address two issues:

1. hard to tell which things are defined in core It's hard to figure out where things are defined #1079,
2. the bootstrap order is confusing -- pyodide_py needs a most of the stuff from core in order to work, but core imports pyodide_py fairly early on in the initialization. Currently it works fine because pyodide_py only needs core to run it's methods not for initialization, but it's easy to make innocuous-seeming changes that mess it up.

Instead of directly adding stuff to pyodide_py, we make a C module called _core and add things to this instead. I also added _core.py which imports names from _core if we are running in the browser and spits out empty shims otherwise. If needed for testing reasons, these shims can be modified. In _core.py we can specify which C source file added each object imported from _core.