Enabled tail-calls, even in Debug mode, for fsharplex -- otherwise the parser causes a StackOverflowException.
Changed the solution configuration so that LegacyInterpreters is always compiled in Release mode.
Renamed the implementation and helper functions for the DIET-specific set functions (union, intersect, difference).
Uncommented the precondition assertions to check the DIET invariant, and added a few more -- it appears that while the DIET mostly works now, there is still a remaining bug or two which causes the invariant to be broken.