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.h

@@ -0,0 +1,53 @@
+/*++
+Copyright (c) 2023 Microsoft Corporation
+
+Module Name:
+
+    euf_arith_plugin.h
+
+Abstract:
+
+    plugin structure for arithetic
+Author:
+
+    Nikolaj Bjorner (nbjorner) 2023-11-11
+
+--*/
+
+#pragma once
+
+#include "ast/arith_decl_plugin.h"
+#include "ast/euf/euf_plugin.h"
+#include "ast/euf/euf_ac_plugin.h"
+
+namespace euf {
+
+    class egraph;
+
+    class arith_plugin : public plugin {
+        enum undo_t { undo_add, undo_mul };
+        arith_util a;
+        svector<undo_t> m_undo;
+        ac_plugin m_add, m_mul;
+        
+    public:
+        arith_plugin(egraph& g);
+
+        ~arith_plugin() override {}
+
+        unsigned get_id() const override { return a.get_family_id(); }
+
+        void register_node(enode* n) override;
+
+        void merge_eh(enode* n1, enode* n2) override;
+
+        void diseq_eh(enode* eq) override {}
+
+        void undo() override;
+
+        void propagate() override;
+        
+        std::ostream& display(std::ostream& out) const override;
+            
+    };
+}