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a lemma function is introduced with 'let lemma f' or 'let rec lemma f'
it is a ghost function
it must have return type unit and read-only effects
it introduces a goal (its WP), followed by an axiom
  forall args/reads, precondition => postcondition

also, fix the evaluation order to left-to-right in e_binop.
Thanks to Johannes for the bug report.

This fixes the bug with .dep cleaning, when we don't build the IDE.

Also, can we merge the replayer into why3session already, please?

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.