- Added general mechanism for model-level evaluation of library
features.

- Renamed some of the methods of interface LibraryFunction.

- Updated ModelLevelExpressionEvaluator to implement the new interface.
- Changed "evaluator" parameters to be typed by the new interface.

- Corrected instantiateInvocation to copy instantiatedType from the
original InvocationExpression, rather than the ownedTyping.

- Updated the evaluation of Features so that unbound input parameters
are considered to have a null value (rather than returning the Feature
itself).