Setup

A tree traverser that prepares a compilation unit to be capture checked. It does the following:

• For every inferred type, drop any retains annotations, add capture sets to all its parts, add refinements to class types and function types. (c.f. mapInferred)
• For explicit capturing types, expand throws aliases to the underlying (pure) function, and add some implied capture sets to curried functions (c.f. expandThrowsAlias, expandAbbreviations).
• Add capture sets to self types of classes and objects, unless the self type was written explicitly.
• Box the types of mutable variables and type arguments to methods (type arguments of types are boxed on access).
• Link the external types of val and def symbols with the inferred types based on their parameter symbols.