Closes https://github.com/cython/cython/issues/3419

The comments are taken from the C API documentation
(https://docs.python.org/3.8/c-api/iterator.html).

Closes #2514.

The comments are taken from the C API documentation
(https://docs.python.org/3.8/c-api/gen.html).

PySlice_Unpack() and PySlice_AdjustIndices() were missing from
slice.pxd.

The comments are taken from the C API documentation
(https://docs.python.org/3.8/c-api/slice.html).

Closes GH-3418.

* Work around a C compiler warning (in PyPy's cpyext).
* Avoid a C compiler warning in PyPy3.
* Fix a C compiler warning about an unused variable when compiling in PyPy.

PyNumber_MatrixMultiply() and PyNumber_InPlaceMatrixMultiply() were
missing from number.pxd.

The comments are taken from the C API documentation
(https://docs.python.org/3.8/c-api/number.html).

* Fixed issue where fused functions didn't register as cyfunctions

Issue https://github.com/cython/cython/issues/3384

This happened where Cython was using a shared API because the
CyFunction type imported from the shared api wasn't the same as
used to initialize the fused function tp_base.

Default arguments are now copied when the bound fused function is created.

Closes #3331.

Closes #1427 

* omp declare privates on outer loop, since inner loops are not 'omp parallel for'ified

They'd generate two calls - one exception checked and one not