Added limit to "visit" to allow detecting multiple visits (#6435)

* Memory leak in .NET user-propagator
The user-propagator object has to be manually disposed (IDisposable), otherwise it stays in memory forever, as it cannot be garbage collected automatically

* Throw an exception if variable passed to decide is already assigned instead of running in an assertion violation

* Added limit to "visit" to allow detecting multiple visits

* Putting visit in a separate class
(Reason: We will probably need two of them in the sat::solver)

* Bugfix

src/util/visit_helper.h

@@ -0,0 +1,39 @@
+#pragma once
+#include "sat_literal.h"
+
+class visit_helper {
+    
+    unsigned_vector         m_visited;
+    unsigned                m_visited_begin = 0;
+    unsigned                m_visited_end = 0;
+    
+    void init_ts(unsigned n, unsigned lim = 1) {
+        SASSERT(lim > 0);
+        if (m_visited_end >= m_visited_end + lim) { // overflow
+            m_visited_begin = 0;
+            m_visited_end = lim;
+            m_visited.reset();
+        }
+        else {
+            m_visited_begin = m_visited_end;
+            m_visited_end = m_visited_end + lim;
+        }
+        while (m_visited.size() < n) 
+            m_visited.push_back(0);        
+    }
+    
+public:
+    
+    void init_visited(unsigned num_vars, unsigned lim = 1) {
+        init_ts(2 * num_vars, lim);
+    }
+    void mark_visited(sat::literal l) { m_visited[l.index()] = m_visited_begin + 1; }
+    void mark_visited(sat::bool_var v) { mark_visited(sat::literal(v, false)); }
+    void inc_visited(sat::literal l) {
+        m_visited[l.index()] = std::min(m_visited_end, std::max(m_visited_begin, m_visited[l.index()]) + 1);
+    }
+    void inc_visited(sat::bool_var v) { inc_visited(sat::literal(v, false)); }
+    bool is_visited(sat::bool_var v) const { return is_visited(sat::literal(v, false)); }
+    bool is_visited(sat::literal l) const { return m_visited[l.index()] > m_visited_begin; }
+    unsigned num_visited(unsigned i) { return std::max(m_visited_begin, m_visited[i]) - m_visited_begin; }
+};