pyximport: fix a bug crashing other processes

pyxbuild overwrites .so file contents, corrupting any mmapped code image
loaded. If pyximporting the same module from two different processes, on
Linux this leads to crashes with bus error.

This commit fixes the issue by unlinking the file first. On Linux et
al., this does not change the file contents of any open FDs.  On
Windows, unlinking fails, and we just switch to using a different file
name.
Add datetime.pxd to cython.cpython. Part II
Add new line at the end of file
Fix inline getters for timedelta
Add test for date/time/datetime/timedelta members
Add missing files
Basic support for Py_UNICODE* strings.
Tests for Py_UNICODE* string support.
MSVC fixes.
Warn on C++ exception declarations for non-extern functions.

This will be made an error.
Fixed explicit coercion of ctypedef'ed C types.
Add function `import_datetime`

Add inline function `import_datetime` for initialization of PyDateTime C API.
Full support for Py_UNICODE[] literals with non-BMP characters.
Change test to initialize by `import_datetime`
Change test to initialize by `import_datetime`
Change test to initialize by `import_datetime`
Fix missing `void`
Fixed a silly typo.
fix reversed order of operands in constant calculation for 'in' and 'not in' operator
remove redundant utility code from ExprNodes.py