Work around error that "__reduce_cython__ cannot be found" when re-importing an extension module that was already initialised before.
Backport to 0.29.x.
Closes #3545.

* Updated URL for the "embedding Python" link in external_C_code.rst
  in the userguide.
* The freenet.org.nz domain has clearly expired at some point and now
  the original link goes spam links, but there are copies in the
  Wayback Machine.
* The updated (fixed) URL uses the most recent archive for the
  original page (author: June, 2004; web archive: Feb, 2012).

generate_type_definitions maintained a list of ctuples for which code
was already generated, but the logic did not match the logic used in
generate_type_header_code, which does not generate code if the entry is
marked as unused.

If the first ctuple of a certain type was marked unused, but later
instances were not, this could result in no code being generated for
the ctuple at all. Resolve by verifying whether the ctuple entry is used
before adding it to the list.
Co-authored-by: Kirk Meyer <kirk.meyer@ravenind.com>

Fixes GH-3534.

Fixes #3531.

* Fixed indexing temps for non-python objects.
* Moved cleanup into release_temps since the temps survived into the result_code.

Closes https://github.com/cython/cython/issues/3430

Prevent calling PyObject_GC_Track() in __Pyx_CyFunction_New() when instantiating a fused function before the object fields of the fused function subtype are fully initialised.
See https://bugs.python.org/issue38392
Closes GH-3215.
Supersedes GH-3216.

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).