Added NE_IS_CONSTANT macro.
NeEnterFrame and NeLeaveFrame are public.
Fixed NeDumpLocals (again!)
Added NE_MEM_CHECK macro and refactored.

NE_MEM_CHECK will cause a runtime error if the expression is 0.
'let' is now native and parallel.
Optimised quoting and fixed fold native.
Added NeQuoteAtom utility and refactored

NeQuoteAtom turns an atom 'atom' into '(quote atom)' if evaluation can change it, otherwise it just returns the original atom.
Added new features to the todo list.