Remove the outdated getbuffer/releasebuffer implementations for the NumPy 'ndarray' since there are probably no NumPy installations out there anymore that do not support the buffer protocol themselves and are still worth supporting.

See https://github.com/numpy/numpy/pull/12284#discussion_r420378155
See GH-3573

Reverse the tuple item order in the utility code caching key to move the most selective item first (just in case if we ever get dict collisions).

See https://github.com/cython/cython/issues/3573

Tighten "numpy_attributes" test to assert that it's actually using a property call and not an attribute access.
Also add a test for the newly added "ndarray.data" property.

Mark the embedded C main() function in the embedding test as "extern C" in C++ to see if that fixes the C++ builds in MSVC.

Fix a C compiler warning about unused cleanup code in buffer using functions that always raise (found by clang in the bufaccess.pyx test).

* Rewrite C property support (GH-2640) based on inline C methods.
Supersedes GH-2640 and GH-3095.
Closes GH-3521.

* Test fix for `numpy_parallel.pyx`: avoid depending on whether "nd.shape" requires the GIL or not.

* Turn NumPy's "ndarray.data" into a property to avoid direct struct access.

* Make "ndarray.size" accessible without the GIL.

Prevent VerboseCodeWriter from inserting nul-bytes into the C file by switching to a more "standard" way of inserting code at a later time.

Improve error handling in "cython.array" creation code to avoid calling Py_BuildValue() with an exception set if "__pyx_format_from_typeinfo()" failed.

Avoid unnecessary int-object creation when formatting numbers as strings in format_from_typeinfo() utility function.

Improve error handling in "cython.array" creation code to avoid calling Py_BuildValue() with an exception set if "__pyx_format_from_typeinfo()" failed.

Refactor step handling in slice_memviewslice() utility code to make it more obvious what is happening in which case.
Closes GH-3568.