whyml: match expression is ghost if we look inside ghost fields
We store in every lsymbol a new integer field ls_constr, equal to zero if the lsymbol is not a constructor, and equal to the number of constructors of the lsymbol's type otherwise. It is allowed to declare or define an lsymbol with ls_constr > 0 as an ordinary function (otherwise algebraic type elimination wouldn't work - though we might still check this in theories), but it is forbidden to use a wrong ls_constr in algebraic type definitions. The ghostness of a match expression is now determined as follows: If at least one branch expression is ghost, then the match is ghost; else if there is only one branch, then the match is not ghost; else if the matched expression is ghost, then the match is ghost; else if at least one pattern matches a ghost field against a constructor with ls_constr > 1 then the match is ghost; else the match is not ghost. We do just enough to recognize obvious non-ghost cases, and make no attempt to handle redundant matches or to detect exhaustive or-patterns in subpatterns.