[Cython] 'with gil:' statement

[mark florisson]

I implemented the 'with gil:' statement, and have added error checking
for nested 'with gil' or 'with nogil' statements. For instance, with
the patch applied Cython wil issue an error when you have e.g.

with nogil:
    with nogil:
        ...

(or the same thing for 'with gil:'). This because nested 'nogil' or
'gil' blocks, without intermediate GIL-acquiring/releasing, will abort
the Python interpreter. However, if people are acquiring the GIL
manually in nogil blocks and then want to release the GIL with a
'nogil' block, it will incorrectly issue an error. I do think this
kind of code is uncommon, but perhaps we want to issue a warning
instead?

The 'with gil:' statement can now be used in the same way as 'with
nogil:'. Exceptions raised from GIL blocks will be propagated if
possible (i.e., if the return type is 'object'). Otherwise it will
jump to the end of the function and use the usual
__Pyx_WriteUnraisable, there's not really anything new there.

For functions declared 'nogil' that contain 'with gil:' statements, it
will safely lock around around the initialization of any Python
objects and set up the refnanny context (with appropriate preprocessor
guards). At the end of the function it will safely lock around the
teardown of the refnanny context. With 'safely' I mean that it will
create a thread state if it was not already created and may be called
even while the GIL is already held (using PyGILState_Ensure()). This
means variables are declared and initialized in the same way as in
normal GIL-holding functions (except that there is additional
locking), and of course the GIL-checking code ensures that errors are
issued if those variables are attempted to be used outside any GIL
blocks.

Could someone review the patch (which is attached)? Maybe check if I
haven't missed any side cases and such?
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