Improve libcpp.unordered_map API

Add zip_safe=False to userguide setup.py

Misc. typos

Closes #2194.

Closes #2193.

This adds notes in the documentation to include zip_safe=False when
using setuptools instead of distutils for setup.py.

Because cimport will not load pxd definitions from a zipped egg file and
the default format when installing from a source package using
setuptools and "python setup.py install" is to create a zipped egg,
including this option will guard against this error when people install
this way.

Found via `codespell -q 3`

Closes #2134.

Added %%cython options.

Patch to add front() and back(), as per issue #2123.

Copy also the short long_neg() code from CPython for the specialised "abs(PyLong)" implementation to avoid jumping through call pointer indirections.

Added argument list for pyximport.

Apply the switch-transformation also recursively to if-clauses and else-body of if-statements that were converted.

Allow `NULL` to be coerced to a pointer to a fused type

Removed dead link to Cython bug tracker.