More tidyups/improvements

 * Untested Cursor.put implementation
 * Remove more unused cffi cdefs (we dont need to cdef functions called
   by helpers)
 * Remove Py_MODINIT_FUNC which was never necessary.
 * Replace class Invalid crap with a simpler/more laborious mechanism.
 * Return _ffi.buffer() instead of Python buffer. Improves performance
   significantly on PyPy, callers can still invoke buffer() as desired.
 * Remove annoying _throw() crap.