BDD variable swapping and reordering

[SHoltzen]

Add a swap method to the BddBuilder trait:

pub trait BddBuilder<'a>: BottomUpBuilder<'a, BddPtr<'a>> {
   /// swaps variables `a` and `b` in the variable order.
   /// all pointers into the manager are invalidated. 
   /// returns a new manager and root list, where the order of roots in the returned list 
   /// matches the order provided by `roots.`
   fn swap(manager: BddManger<'a>, roots: Vec<BddPtr<'a>, a: VarLabel, b: VarLabel) -> (Manager<'b>, Vec<BddPtr<'b>>;

   /// reorders all BDDs to have the order `order`.
   /// all pointers into the manager are invalidated. 
   /// returns a new manager and root list, where the order of roots in the returned list 
   /// matches the order provided by `roots.`
   fn reorder(manager: BddManger<'a>, roots: Vec<BddPtr<'a>, order: VarOrder) -> (Manager<'b>, Vec<BddPtr<'b>>;
}

• Swapping adjacent variables in a BDD is efficient. Swapping two arbitrary variables can be accomplished by repeated swapping of adjacent variables.
• Here is a document visualizing swapping and describing how to efficiently implement it:
  variable-swap.pdf
• reorder should be implemented in terms of swap.
• This is blocked by Garbage collection #154.