Fix parallel assignment in the Executor.
Fix parallel assignment in the Executor.

We need to store assignments rather apply them immediately
so we implement parallel assign

x, y = y, x;

like this

t0 := y;
t1 := x;
x := t0;
y := t1;

rather than

x := y;
y : x;

which is wrong and does not follow Boogie's semantics (the parallel
assignments should not see the effect of each other). See 9.3 of "This is Boogie 2".

This fixes the broken Assignment.ParallelSymbolicAssignment() test case.

symbooglix/symbooglix,symbooglix/symbooglix,symbooglix/symbooglix