Use cffi's new out-of-line mode for faster CPython #5

CFFI 1.0 introduced a new out-of-line mode to reduce the overhead when running on CPython.

See:

• http://morepypy.blogspot.com/2015/05/cffi-101-released.html
• https://cffi.readthedocs.org/en/latest/overview.html#out-of-line-example-abi-level-out-of-line

(Wishlist item rather than bug.)