Passed the engine instance to loaders. This is a prerequisite for
looking up configuration on the engine instance instead of global
settings.
This is backwards incompatible for custom template loaders that override
__init__. However the documentation doesn't talk about __init__ and the
way to pass arguments to custom template loaders isn't specified. I'm
considering it a private API.
Since it isn't branded as a test utility any more and could be used for
other purposes than test code, that prefix no longer makes sense.
It wasn't used anywhere either.
Reformatted the code of base.Loader according to modern standards.
Turned the test template loader into a regular locmem.Loader -- but
didn't document it.
Added a normal deprecation path for BaseLoader which is a public API.
Added an accelerated deprecation path for TestTemplateLoader which is
a private API.