Module usage

Introduces Usages, which is responsible for computing variables usage in semantic blocks
of a function.

Structs

• Usage
  Usages of variables and member paths in semantic code.
• Usages
  Usages of member paths in expressions of interest, currently loops and closures.

Enums

• MemberPath
  Member path (e.g. a.b.c). Unlike ExprVarMemberPath, this is not an expression, and has no syntax pointers.