unexpected feedback from type-checker about unbound variables in return type of ParseTree::parsers

[jurgenvinju]

• I suspect the real error is that one can not and should not introduce fresh type variables on the left-hand side of an assignment
  • only left-hand side of pattern matches := and formal parameters of functions, case patterns can introduce type variables.
• the &U from the definition of parsers is open in the function return type; it looks from the current error message that this &U is deemed to be closed. Solving this by itself could also resolve this issue without focusing on whether or not assignments can introduce type variables.