An alias-based formulation of the borrow checker (2018)

Ever since the Rust All Hands, I've been experimenting with an alternative formulation of the Rust borrow checker. The goal is to find a formulation that overcomes some shortcomings of the current proposal while hopefully also being faster to compute. I have implemented a prototype for this analysis. It passes the full NLL test suite and also handles a few cases - such as #47680 - that the current NLL analysis cannot handle. However, the performance has a long way to go (it is currently slower than existing analysis). That said, I haven't even begun to optimize yet, and I know I am doing some naive and inefficient things that can definitely be done better; so I am still optimistic we'll be able to make big strides there.