Somewhat nicer to use and guards against static init order fiasco.

It is used wherever in WCCL variable is seen in read-only context, i.e. outside of operators that are setting its value like setvar.

Generally we want to avoid accidental change of value of a constant or of a variable. Originally I'd copy value on access, but this changed it so a const Value is returned instead and copy is done only when it's needed.

Functions operate on it now instead of on just SentenceContext. It's open to additions in future. Main purpose atm is to hold Variables as well as SentenceContext.
Done some cleanup in code too. Updated tests to reflect the FunExecContext change as well.

Making the default for "value-if-false" parameter to be a static, initialized on demand, member of Conditional.

Conditional operators realised as if..then..else expression and operator "? val ? pred" with some tests for StrSet specialization

Adding specialization of Equals for positions, as they logically need to take into account actual sentence context for full comparison.

LogicalPredicate, abstract base class for predicates that are logical functions taking any number of arguments