Updated roadmap for closures and fn pointers

[cdstanford]

New roadmap for closures and function pointers. Replaces #30

For v0:

• add two types of unsafe effect to our effect model: ClosureCreation and FnPointerCreation
• in scanner.rs, whenever a closure or function pointer is created, add this to the list of effects associated with that function that need to be audited.

For v1, the plan is probably something like the following:

• add closures to the call graph
• for effects in a closure, we need a new audit model: effects can be caller-checked, creator-checked, safe, or unsafe.
• for each closure variable or function pointer variable, track which possible closures/function pointers it might refer to (an overapproximation). If we don't have enough information to identify, could add an UnsafeCall effect to track this.

[cdstanford]

• Currently in data/results/top10_all.csv, we have several CLOSURE lines, these should eventually go away for the roadmap above.

[cdstanford]

Limitations of current FnPtrCreation tracking:

Currently it doesn't track type aliases, statics, unnamed fields, and function pointers in smart pointers.

[cdstanford]

Just discussed this with David, false positives for closure creation seem to come up commonly, e.g. for closures like |x| x + 1 which are quite common.

An extremely useful optimization we should do here is:

• if the closure contains no effects inside it -- so absolutely no effects are possible when called -- skip adding the ClosureCreation effect.