Rewrite _unellipsify() helper function in memoryview code to speed it up and reduce the generated C code.

See https://github.com/cython/cython/pull/3562

See #1407.

These are mutable and therefore affect the stdlib build_ext options.

Fixes #2209

See #1407.

This reverts commit 4900109c.
This is trying to parallelise the appveyor build again after allowing the setup.py script to be imported on a multiprocessing "spawn".

Make the setup.py script importable to enable multiprocessing builds that need to "spawn" processes (usually on Windows).

Fix duplicate code generation in assert condition, found for lambda functions that are used as conditions in "test_grammar.py".

These are mutable and therefore affect the stdlib build_ext options.

Fixes #2209

This was previously broken by the introduction of the "__PYX_ERR()" macro.