add explicit move constructor to deal with unit test regression test-…

…z3 algebraic on Windows/debug -

 it uses copy constructor instead of move when returning a scoped_anum in functions such as power and root.
This leads to freeing memory that gets passed as return value.

The copy constructor for scoped_numeral is also suspicious because it doesn't ensure memory ownership.

src/util/scoped_numeral.h

@@ -28,8 +28,8 @@ class _scoped_numeral {
     numeral   m_num;
 public:
     _scoped_numeral(Manager & m):m_manager(m) {}
-    _scoped_numeral(_scoped_numeral const & n):m_manager(n.m_manager) { m().set(m_num, n.m_num); }
-    _scoped_numeral(_scoped_numeral &&) = default;
+    _scoped_numeral(_scoped_numeral const& n) :m_manager(n.m_manager) { m().set(m_num, n.m_num); }
+    _scoped_numeral(_scoped_numeral && n) noexcept: m_manager(n.m_manager) { m().swap(m_num, n.m_num); }
     ~_scoped_numeral() { m_manager.del(m_num); }
 
     Manager & m() const { return m_manager; }