Lifter refactor and scope fix

[CAG2Mark]

A long overdue refactor of the lifter and support for scopes. This will probably take a while.

I also plan on doing the following:

• The data structure used to store certain data about the functions uses space that is quadratic in the nesting depth anyway, so I will probably try to implement a more precise analysis on which variables are required by which functions. This analysis will take quadratic time. The lifting itself will still only rewrite the IR blocks O(1) times.
• Separate lifting into two passes: rewriting definitions to be independent of their current scope, and the actual lifting. I vaguely recall we wanted to only do the first pass at some point, so the ability to turn the "physical" lifting off may be useful.