Cpp copy added to libcpp/algorithm.pxd

Merge upstream

Fix typos.

s/compatability/compatibility/g

fix cascaded assignments to buffer indices: forgot to reassign the analysed (and potentially replaced) node in the tree

These would (and should) probably be inlined regardless.

Allow output file to be a dir. E.g. 'cython -o . /path/somefile.pyx'

Conflicts:
	CHANGES.rst
	Cython/Shadow.py

Fix a memory leak in ObjectHandling.c

some more minor fixes to the buffer docs

Python docs says they should not be used in extension modules.
https://docs.python.org/3/c-api/gcsupport.html#c._PyObject_GC_TRACK