Moved speedups to a separate package.

This pull request is designed first and foremost as a springboard for a discussion about a possible alternative to using the setuptools Feature functionality. The change extracts the C extensions into a SQLAlchemy-speedups project (which I've temporarily hosted at [jaraco/sqlalchemy-speedups](/jaraco/sqlalchemy-speedups) and published to PyPI). This technique allows the extensions to be indicated by requiring SQLAlchemy[speedups] or by installing the speedups package explicitly.

I am encouraged by this change. It was relatively easy to enact and it makes the code much cleaner. Suddenly, SQLAlchemy is a pure-Python package with much simpler build and packaging semantics, yet with a straightforward way to achieve the performance gains. It also means that the speedups package can be released and distributed separately, possibly as wheels or eggs or other platform-specific formats and only that package need deal with the complexities of the compile and build process.

Furthermore, by decoupling the aspects as separate packages, a deployment manager can easily detect the presence or absence of the speedups (even outside of the Python ecosystem), which would be harder to do with the aspect selected at build time.

I acknowledge that this change does require a bit of shift in perception about how SQLAlchemy is referenced and installed, and that's a non-trivial shift. That is, there's no way for the 'speedups' extra to be indicated by default and specifically contra-indicated (opt-out). Therefore, implementers would need to be instructed to always include [speedups] unless they were desired to be omitted.

I do believe that following this technique simplifies the build process and does move the packaging in a direction that's more consistent with the simpler build processes being developed by the PyPA (though I couldn't say specifically why; I only suspect based on the essence of the decoupling).

I'm eager to hear your feedback on this approach. Is it viable for SQLAlchemy? If so, what sort of timeline might one expect for adoption?

You're welcome to accept this pull request or re-implement the concept in your own style. I'm also happy to give up ownership of the SQLAlchemy-speedups name if that's the name you choose to adopt. I merely wanted to draft a fully-functional prototype to aid in your consideration.