Added to each Context a reference to the Engine.

It's only available during the rendering.
Move compile_string into the Engine class.
Moved make_origin into the Engine class.
Encapsulated ALLOWED_INCLUDE_ROOTS in Engine.
Encapsulated TEMPLATE_CONTEXT_PROCESSORS in Engine.

Since RequestContext doesn't know its Engine until it's passed to
Template.render() -- and cannot without breaking a widely used public
API -- an elaborate hack is required to apply context processors.
Encapsulated TEMPLATE_STRING_IF_INVALID in Engine.
Encapsulated TEMPLATE_DEBUG in Engine.
Removed direct references to template-related settings.