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.