parallel build support for ffibuilder.compile()

[James-E-A]

Is there any real reason this wasn't added yet?

from cffi import FFI

ffibuilder = FFI()
...
# pass parallel=n to use that many threads,
# or parallel=True to automatically use os.cpu_count()
# https://github.com/python/cpython/blob/v3.5.0/Lib/distutils/command/build_ext.py#L449
# https://github.com/pypa/setuptools/blob/v66.1.0/setuptools/_distutils/command/build_ext.py#L470
ffibuilder.compile(verbose=True, parallel=True)

[arigo]

Looks reasonable to me. It's missing tests and documentation for all public methods with the new parallel keyword argument. (For tests I'd be happy with just making sure there is at least one test somewhere that gives some value to parallel, and one that doesn't, and both passes.)