The Equals<T> now uses helper class EqualityComparer<T> and distinction is in the helper class specialisation.

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

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.

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