Add bootstrap-fast.php and make it the default.

Our current --bootstrap requirement when running tests adds considerable time to the start of each test run. When you are developing or debugging a single test, its frustrating.

This PR adds a bootstrap-fast.inc and recommends it over bootstrap.inc which stays in the repo, for those who want it. bootstrap-fast.inc skips all directory scanning and just registers the namespaces that our traits use. This is very fast. Users are welcome to customize bootstrap-fast.inc as needed.

I'm open to other ways of handling this, and better ways to document it.