Change semantics of cprover::Assign to _

_ is a dummy variable. We now ignore assignments to _. Earlier, we used to
havoc all variables in scope.
[Major Change] Bug fix in pds generation of parallel assign in cprover.

Parallel should existentially quantify out the variable that is being assigned
to. This was not the case.
Large BddNode table initialization.

Had to switch off tcmalloc because of known bug in tcmalloc.