Updated roadmap for closures and fn pointers

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.

Start by reading scanner.rs and the existing effect model, then compare the completed v0 checklist with the proposed v1 work. The issue describes adding closures to the call graph, tracking possible closure or function-pointer targets, and defining a new audit model, but it does not specify a bounded implementation task or clear completion test.