add EUF plugin framework.

plugin setting allows adding equality saturation within the E-graph propagation without involving externalizing theory solver dispatch. It makes equality saturation independent of SAT integration.
Add a special relation operator to support ad-hoc AC symbols.

src/ast/euf/euf_justification.cpp

@@ -0,0 +1,54 @@
+/*++
+Copyright (c) 2020 Microsoft Corporation
+
+Module Name:
+
+    euf_justification.cpp
+
+Abstract:
+
+    justification structure for euf
+
+Author:
+
+    Nikolaj Bjorner (nbjorner) 2020-08-23
+
+--*/
+
+
+#include "ast/euf/euf_justification.h"
+#include "ast/euf/euf_enode.h"
+
+namespace euf {
+
+
+    std::ostream& justification::display(std::ostream& out, std::function<void (std::ostream&, void*)> const& ext) const {
+        switch (m_kind) {
+        case kind_t::external_t:
+            if (ext)
+                ext(out, m_external);
+            else
+                out << "external";
+            return out;
+        case kind_t::axiom_t:
+            return out << "axiom";
+        case kind_t::congruence_t:
+            return out << "congruence";
+        case kind_t::dependent_t: {
+            vector<justification, false> js;                
+            out << "dependent";
+            for (auto const& j : dependency_manager::s_linearize(m_dependency, js))
+                j.display(out << " ", ext);
+            return out;
+        }
+        case kind_t::equality_t: 
+            return out << "equality #" << m_n1->get_id() << " == #" << m_n2->get_id();
+            
+        default:
+            UNREACHABLE();
+            return out;
+        }
+        return out;
+    }
+
+}