Implement scan key qualification.

The type system should eventually be pluggable, but for now it's
a fixed implementation based on Go's interface.  Still looking to see
how it should be written in this language.
Fix unbuilt part due to the recent change.
Create Alias for parser.

This should make a minimal parser to parse Var and RangeVar.  Expr is
now an interface, although it is still not clear to me how to represent
this kind of abstract super type.