Implement features that should help avoid memory problems.
UL4Repr.Formatter no longer calls the visit method in the constructor.

This makes it possible to subclass UL4Repr.Formatter for special output.
To use the Formatter use the following code:

    new UL4Repr.Formatter().visit(obj).toString()
Avoid duplicate pop operation in error case.
An InterpretedTemplate can now have a signature. Calling or rendering the
template will now check the variables passed in against the signature.
Add missing class.
Whitespace.
Implement all variants of the EvaluationContext constructor. Remove getAllVariables.
Remove ancient code that never served any purpose.
Add signatures (no signatures for sub template yet).
Add support for signatures on sub templates.