Visualizing outlives-constraints and lifetime parameters

[willcrichton]

To talk about borrow checker errors involving lifetime parameters, we designed a "flow permission" that is distinct from read/write/own (RWO). See this section of the Rust Book Experiment for explanation: https://rust-book.cs.brown.edu/ch04-02-references-and-borrowing.html#data-must-outlive-all-of-its-references

This flow permission is unsatisfactory because, unlike RWO, it doesn't have a "state" that can be visualized. The issue is that we don't have any way to talk about the outlives-constraints in the program, namely the outlives-constraints between function parameters, and their relation to outlives-constraints induced in the function body.

We are interested in either refining or replacing the flow permission. The ultimate goal of the refinement/replacement is to help Rust users understand how lifetime parameters work.