working on #5614

there are some different sources for the performance regression illustrated by the example. The mitigations will be enabled separately:
- m_bv_to_propagate is too expensive
- lp_bound_propagator misses equalities in two different ways:
   - it resets row checks after backtracking even though they could still propagate
   - it misses equalities for fixed rows when the fixed constant value does not correspond to a fixed variable.

FYI @levnach

src/math/lp/lar_term.h

@@ -155,7 +155,7 @@ public:
     };
     
     class const_iterator {
-        u_map< mpq>::iterator m_it;
+        u_map<mpq>::iterator m_it;
     public:
         ival operator*() const { return ival(m_it->m_key, m_it->m_value); }        
         const_iterator operator++() { const_iterator i = *this; m_it++; return i;  }