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/CMakeLists.txt

@@ -2,6 +2,7 @@ z3_add_component(polysat
   SOURCES
     boolean.cpp
     clause_builder.cpp
+    conflict_core.cpp
     constraint.cpp
     eq_constraint.cpp
     explain.cpp