Create only a single FFI, with one call to verify().

[arigo]

This calls ffi.verify() only once, and avoids calls to ffi.include().

[arigo]

Note that running all tests with "test.py" hangs after a while for me (at the same place before and after my commit), so that I could only check in part that I really didn't break anything.

[mithrandi]

This change is now merged into https://github.com/lxml-cffi/lxml-cffi (in preparation for switching to the new set_source API); the test failure/hang is still there, unfortunately.

EDIT: The tests are passing on lxml-cffi now, for any lost souls that landed up here.