msvc: Build seperate micropython dll and executable

- export some core functions using exports.def, export constants/data using the
typical dllexport (exporting data using .def file yields multiple copies of data).
- rename micropython project to micropythoncore and have it produce a dll + import lib
- add new micropython exe project (which just calls the exported main from the dll)
- add run.h/run.c which allow for easy embedding
- add property sheet for external modules