This removes the stateful Pipeline object and leaves behind only a toolbox
of functions for constructing element lists, such as _pipeline.get_selection()
and _pipeline.except_elements(), and some helpers for asserting element states
on lists of elements.

This makes it easier for Stream to manage it's own internal state, so that
Stream can more easily decide to operate without hard requiring a Project
instance be available.

This also adds type annotations to the new version of _pipeline.py.