Version 0.0 of a "ground evaluation" library.

Uses kernel's lazy beta, but doesn't use its clever Mk_comb
alternative (which saves allocations).  Handles rewrites, and should
extend to conversions for which users can predict "value-shape" of
result.  No special treatment of lazy case constants.

Also, excessive tracing at the moment.
Split groundEval regression tests out into selftest.sml.

Also add placeholder for treatment of case constants (which don't
evaluate all their arguments).