support libffi (and ctypes)

[williamstein]

See:

• https://github.com/hoodmane/libffi-emscripten
• https://discuss.python.org/t/get-ctypes-working-in-emscripten-build/18466

[williamstein]

I've added PARTIAL support, but put the EM_JS stuff as stubs for now. This means that obviously some functionality won't work. This is just a tiny step so all those random scripts that import ctypes and do nothing with it (it's usually platform specific stuff on macos or windows) will at least not crash.

[williamstein]

Example:

>>> import ctypes
>>> ctypes.pythonapi["PyObject_Str"]()
STUB: ffi_call
0

There is nothing difficult about fully supporting this properly. It's just a matter of organization and understanding the hard work already done by https://github.com/hoodmane/libffi-emscripten

Thanks!

[pmp-p]

afaik the work is documented here
https://brionv.com/log/2018/05/06/emscripten-fun-porting-libffi-to-webassembly-part-1/
https://brionv.com/log/2018/05/27/emscripten-fun-libffi-on-webassembly-part-2/