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_plugin.h

@@ -0,0 +1,58 @@
+/*++
+Copyright (c) 2023 Microsoft Corporation
+
+Module Name:
+
+    euf_plugin.h
+
+Abstract:
+
+    plugin structure for euf
+
+    Plugins allow adding equality saturation for theories.
+
+Author:
+
+    Nikolaj Bjorner (nbjorner) 2023-11-08
+
+--*/
+
+#pragma once
+
+#include "ast/euf/euf_enode.h"
+#include "ast/euf/euf_justification.h"
+
+namespace euf {
+
+    
+    class plugin {
+    protected:
+        egraph& g;
+        void push_plugin_undo(unsigned th_id);
+        void push_merge(enode* a, enode* b, justification j);
+        void push_merge(enode* a, enode* b);
+        enode* mk(expr* e, unsigned n, enode* const* args);
+        region& get_region();
+    public:
+        plugin(egraph& g):
+            g(g)
+        {}
+
+        virtual ~plugin() {}
+
+        virtual unsigned get_id() const = 0;
+
+        virtual void register_node(enode* n) = 0;
+        
+        virtual void merge_eh(enode* n1, enode* n2) = 0;
+
+        virtual void diseq_eh(enode* eq) {};
+
+        virtual void propagate() = 0;
+
+        virtual void undo() = 0;
+        
+        virtual std::ostream& display(std::ostream& out) const = 0;
+            
+    };
+}