Fix bug in scoping for default arguments

[janmasrovira]

when we have:

f {arg : Nat := X} ...

then arg should not be in scope in X.

Before this pr, if arg was used in X, we got an impossible error in the typechecker.
E.g. typechecking the following results in a crash. After this pr, it gives a scoping error.

module bug;

type T := mkT;

f {x : T := x} : T := mkT;