Polysat updates (#5444)

* Simplify adding lemmas

* Remove misleading constructor from tmp_assign.

The idea is that tmp_assign is only created on the stack and
short-lived.  Instead of having a convenience constructor that takes a
constraint_ref, it's clearer to have an explicit .get() at the call
site.

* Remove some log messages

* bugfix

* fix

* Add stub for conflict_core

* wip

* Add example by Clemens

src/math/polysat/conflict_core.cpp

@@ -0,0 +1,28 @@
+/*++
+Copyright (c) 2021 Microsoft Corporation
+
+Module Name:
+
+    polysat conflict
+
+Author:
+
+    Nikolaj Bjorner (nbjorner) 2021-03-19
+    Jakob Rath 2021-04-6
+
+--*/
+
+#include "math/polysat/conflict_core.h"
+#include "math/polysat/solver.h"
+#include "math/polysat/log.h"
+#include "math/polysat/log_helper.h"
+
+namespace polysat {
+
+    std::ostream& conflict_core::display(std::ostream& out) const {
+        out << "TODO: display conflict_core";
+        // depending on sign:   A /\ B /\ C     or     ¬A \/ ¬B \/ ¬C
+        return out;
+    }
+
+}