Lazy partial evaluation

A function for evaluating any term to weak-head normal form.

A function for "performing" an HMeasure monadic action. This could mean actual random sampling, or simulated sampling by generating a new term and returning the newly bound variable, or anything else.

A function for evaluating any case-expression to weak-head normal form.

A function for evaluating any variable to weak-head normal form.

Lazy partial evaluation with some given "perform" and "evaluateCase" functions. The first argument to evaluateCase will be the TermEvaluator we're constructing (thus tying the knot). N.B., if p ~ 'Pure then the "perform" function will never be called.

We factor out the CaseEvaluator because some code (e.g., disintegration) may need to do something special rather than just relying on the defaultCaseEvaluator implementation.

A simple CaseEvaluator which uses the DatumEvaluator to force the scrutinee, and if matchBranches succeeds then we call the TermEvaluator to continue evaluating the body of the matched branch. If we GotStuck then we return a Neutral term of the case expression itself (n.b, any side effects from having called the DatumEvaluator will still persist when returning this neutral term). If we didn't get stuck and yet none of the branches matches, then we throw an exception.