fix for unreachable yield in generators

--HG--
extra : rebase_source : aca82bcaf6b2e80519f6ed0073ce7164385e5a5c

There are a large number of constants declared with 'cdef enum' in
numpy.pxd. Most of them did not correctly match the definitions in the
numpy headers -- either because of a missing 'ctypedef', or because
numpy.pxd claimed that they were of a named enum type ('enum
requirements') which did not actually exist.

Somewhere between Cython 0.12.1 and 0.14.1, the latter issue broke my
compile whenever I referred to np.NPY_F_CONTIGUOUS. Cython started
generating code like:
  enum requirements __pyx_t_10 = NPY_F_CONTIGUOUS;
  PyArray_NewFromDescr(..., __pyx_t_10, ....);
Since there is, in fact, no such thing as 'enum requirements', the C
compiler objected. (In earlier version, no temporary variable was
generated, so we got away with the incorrect declaration.)

And since I had to fix the declaration of this particular enum, I
figured I might as well double-check the rest as well, in case of
similar problems arising in the future.

With this patch, Cython can once again compile scikits.sparse v0.1.

Unreachable code removal

- iteration over empty dicts could crash
- dict modification during iteration failed to raise an exception

according to unladen swallows' bm_unpack_sequence benchmark:
- ~25% faster for tuples (60% faster than CPython 2.7)
- ~3x faster for lists (40% faster than CPython 2.7)
also, apparently using less memory during gcc compilation

--HG--
extra : rebase_source : 1069d7a0c9c338c16e96cc289114e0d0052a7588

--HG--
extra : rebase_source : 18e87455b812d7b9a1f2f2b28593455ca5521e89