Get the size of an AST node

[JonathanSalwan]

In several cases (when we perform simplification) we must know the size of the root BVNODE. To do that, we must compute the AST. (c.f: concat, extract, bv, sx, zx).

The method node.getBitvectorSize() returns the current size of the root node. Which means that we can now perform simplification like this:

/* if (bvxor x x) -> (_ bv0 x_size) */
smt2lib::smtAstAbstractNode* xor_simplification(smt2lib::smtAstAbstractNode* node) {

  if (node->getKind() == smt2lib::BVXOR_NODE) {
    if (*(node->getChilds()[0]) == *(node->getChilds()[1]))
      return smt2lib::bv(0, node->getBitvectorSize());
  }

  return node;
}

Related to #226 and #93. Done with the next v0.3.