Ensure that types derived for REPL expressions have no free type variables

[julianhyde]

Ensure that types derived for REPL expressions have no free type variables. Currently the type resolver will return a type like 'a list -> 'a, which has a free type variable. When that value is applied to a value, say an int, the type of the result will be wrongly given as 'a when it should be int.

The solution is to wrap the derived type in a ForallType if there are any free type variables.

As part of the solution, add class TypeVisitor and method Type.accept(TypeVisitor).

[julianhyde]

Fixed in 8a553d8.