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_arith_plugin.cpp

@@ -0,0 +1,71 @@
+/*++
+Copyright (c) 2023 Microsoft Corporation
+
+Module Name:
+
+    euf_arith_plugin.cpp
+
+Abstract:
+
+    plugin structure for arithetic
+
+Author:
+
+    Nikolaj Bjorner (nbjorner) 2023-11-11
+
+--*/
+
+#include "ast/euf/euf_arith_plugin.h"
+#include "ast/euf/euf_egraph.h"
+#include <algorithm>
+
+namespace euf {
+
+    arith_plugin::arith_plugin(egraph& g) :
+        plugin(g),
+        a(g.get_manager()),
+        m_add(g, get_id(), OP_ADD),
+        m_mul(g, get_id(), OP_MUL) {
+        std::function<void(void)> uadd = [&]() { m_undo.push_back(undo_t::undo_add); };
+        m_add.set_undo(uadd);
+        std::function<void(void)> umul = [&]() { m_undo.push_back(undo_t::undo_mul); };
+        m_mul.set_undo(umul);
+    }    
+
+    void arith_plugin::register_node(enode* n) {
+        // no-op
+    }
+
+    void arith_plugin::merge_eh(enode* n1, enode* n2) {
+        m_add.merge_eh(n1, n2);
+        m_mul.merge_eh(n1, n2);
+    }
+
+    void arith_plugin::propagate() {
+        m_add.propagate();
+        m_mul.propagate();
+    }
+
+    void arith_plugin::undo() {
+        auto k = m_undo.back();
+        m_undo.pop_back();
+        switch (k) {
+        case undo_t::undo_add:
+            m_add.undo();
+            break;
+        case undo_t::undo_mul:
+            m_mul.undo();
+            break;
+        default:
+            UNREACHABLE();
+        }
+    }
+        
+    std::ostream& arith_plugin::display(std::ostream& out) const {
+        out << "add\n";
+        m_add.display(out);
+        out << "mul\n";
+        m_mul.display(out);
+        return out;
+    }
+}