ENH Add Ctypes support

[hoodmane]

Resolves #728.

I took the libffi port started by @Brion and added support for structs and closures (and fixed some issues with long double). I have that code here:
https://github.com/hoodmane/libffi-emscripten

It currently passes 56/56 of the "call" tests and 84/94 of the "closures" tests. It would be good to set up continuous integration for the library and to upstream it into the emscripten patches.

[rth]

Very excited about the progress you have been making on this @hoodmane !

[hoodmane]

It built!! Finally!

[hoodmane]

Uh-oh now it's failing with ModuleNotFoundError: No module named '_socket'.

[hoodmane]

Indeed for some reason it didn't build a bunch of built in modules.

[hoodmane]

Okay the build in 27df231 actually worked and a simple ctypes use works:

>>> import ctypes
>>> tuple_size = ctypes.CFUNCTYPE(ctypes.c_long, ctypes.py_object).in_dll(ctypes.pythonapi, "PyTuple_Size")
>>> tuple_size((1,2,3))
3

https://6515-322074228-gh.circle-artifacts.com/0/root/repo/build/console.html

[hoodmane]

The CoExtra test fails because PyCode_SetExtra doesn't seem to work at all. Using it corrupts memory. Not ctype's fault. cpython_core[test_ctypes] passes on both Firefox and Chrome. Still have to investigate why CLAPACK build is failing...

[hoodmane]

Okay so cpython_core[test_unicode-chrome] is failing for some reason and CLAPACK build is failing for some reason but other packages build and pass tests, even with ctypes patches removed. So we're in pretty good shape here.

[hoodmane]

Right so test_unicode uses ctypes to do a bunch of tests for C APIs when ctypes is available. Probably it's again an issue with one of the C string APIs just like the code test was failing because PyCode_SetExtra is bad.

[hoodmane]

I think the problem with test_unicode is that _PyUnicode_FromFormat is variadic. I'm not sure how to make call variadic functions from Javascript.

[hoodmane]

@rth Any clue what could be causing the CLAPACK build failure on 3f2baf4? The error message is pretty not helpful. I guess CLAPACK doesn't link Python and it doesn't take that long to fail so it should be easy to bisect.

[hoodmane]

Okay it broke between 66400d0 and 426f5b2. I guess that emsdk patch causes trouble -- those optimization passes I dummied out are probably necessary for the CLAPACK build to succeed. Luckily I can remove the patch because I found out it is caused by the 123n syntax for a BigInt so I can remove uses of that from the libffi implementation and we should be good to go.
emscripten-core/emscripten/issues/14525

[hoodmane]

Well removing that patch definitely fixes the CLAPACK build. But removing the bigint n's didn't fix the linking.

[rth]

Looks like you found the solution, so I'm not going to be of much help :)

[hoodmane]

Scipy imports are failing. I guess I'll restore the patch.

[hoodmane]

Okay on 281c721 all tests passed except for two typical chrome-packages timeouts.

[rth]

Great work @hoodmane !

[rth]

Suggested change

	- Added support for `ctypes`.
	- Added support for the [`ctypes`](https://docs.python.org/3/library/ctypes.html)
	stdlib module.