On N121-024 we fixed a bug where a state was incorrectly reused during
merge. The fix was to never reuse state when merging. This resulted in a
performance drop.

The new fix now marks states which should not be reused - basically
states that come from raise - and when such a state is present during
merging, we don't reuse. Otherwise, we reuse.  This seems to get us the
performance and passes all tests.

Marking is implemented simply by a flag in states

* mlw_wp.ml:
(mark) allows to set the marked flag of a state
(empty) empty state is not marked
(havoc) havoc doesn't change marked state
(merge_vars, merge_regs) new argument "marked", only create a new
  variable when marked is true, otherwise reuse
(fast_wp) in the case of Eraise, mark the used state

M415-042 pass labels down to conjunctions

Why3 parser groups the loop invariants into a single formula. This patch
allows to have the VC labels on every conjunct

mlw_wp.ml
(fastwp) call wp_expl instead of t_label