Move CYTHON_UNUSED macro up such that included files can use it

Closes #1506

Closes #1506

Fixes #1499

This gives projects more time to move over, and reduces the number
of changes in the 0.25 series.

Fixes #1502

Note about directive defaults.

Closes #1496

Closes #1493

This partially reverts 65d3b977,
but is more correct.

Closes #1492

Implement numpy's import_array and friends in Cython

Add declarations for longintrepr.h

Revert "test: assert that isinf returns -1 for negative infinity"

This reverts commit 24cf7c96.

According to the numpy documentation npy_isinf merely provides an
equivalent of C99 isinf, which in turn is only required to return a
non-zero value for a given infinite argument.

The original commit breaks the test when compiled with clang & libc++.