GetCallStubInfo extracts necessary information to generate a method definition from a CallExpr.

GetIfaceStubInfo determines whether the "missing method error" can be used to deduced what the concrete and interface types are.

CallStubInfo represents a missing method that a receiver type is about to generate which has "type X has no field or method Y" error.

IfaceStubInfo represents a concrete type that wants to stub out an interface type.