fix creation order of set literals by first generating all items and only then adding them to the set (as CPython does it)

--HG--
rename : tests/run/set.pyx => tests/run/set_literals.py

fused types: specialize each base type only once, also for compound type args

--HG--
extra : amend_source : 04828c16faf5af6bf7203a7ae59ad1f365cb5699

be more informative about current Python version when it's too old (might be automatically selected by shebang)

Conflicts:
	CHANGES.rst
	Cython/Shadow.py

Add more posix.unistd constants

Add posix constants for _CS_*, _PC_* and _SC_*.

Bugs noticed by Ami Tavory.

Fix crossref to compiler directives section

Rewrote the 'Pure Python mode' tutorial