Self-referential generic types require forward declarations

[Rob Schluntz]

The parser does not recognize generic types until the ending semicolon.

forall(otype T)
struct List {
  List(int) * x;  // parsing error
};

The current workaround is to forward declare generic types.

forall(otype T)
struct List;

forall(otype T)
struct List {
  List(int) * x;  // okay, forward declaration makes List a generic type
};