Investigate Polonius

[DieracDelta]

We'd like to model a subset of borrow rules for Rust. Candidates for determining what that logic should look like could come from either the Rust source (polonius) or Stack/Tree borrows. I considered the ferrocene spec, but their borrow checking section is incomplete.

Questions to answer:

• How are *mut pointers modeled? What rules are used?
• How are *const pointers modeled? What rules are used?
• What is the minimum set of behavior/rules can use to model *mut* and *const*
• Can we mesh these rules with the region-based approach taken by oxide?