work around bug in fused types code that prevents typing the signature of the runtime dispatch function

fix test: runtime dispatch is done only on the first occurrence of a fused buffer type and later mismatches lead to type failures after the call (which is acceptable since it speeds up the dispatch and leads to an error anyway)

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