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.cpp

@@ -0,0 +1,47 @@
+/*++
+Copyright (c) 2023 Microsoft Corporation
+
+Module Name:
+
+    euf_plugin.cpp
+
+Abstract:
+
+    plugin structure for euf
+
+    Plugins allow adding equality saturation for theories.
+
+Author:
+
+    Nikolaj Bjorner (nbjorner) 2023-11-08
+
+--*/
+
+#include "ast/euf/euf_egraph.h"
+
+namespace euf {
+    
+    void plugin::push_plugin_undo(unsigned th_id) {
+        g.push_plugin_undo(th_id);
+    }
+
+    void plugin::push_merge(enode* a, enode* b, justification j) {
+        g.push_merge(a, b, j);
+    }
+
+    void plugin::push_merge(enode* a, enode* b) {
+        TRACE("plugin", tout << g.bpp(a) << " == " << g.bpp(b) << "\n");
+        g.push_merge(a, b, justification::axiom());
+    }
+
+    enode* plugin::mk(expr* e, unsigned n, enode* const* args) {
+        enode* r = g.find(e);
+        if (!r)
+            r = g.mk(e, 0, n, args);
+        return r;
+    }
+
+    region& plugin::get_region() {
+        return g.m_region;
+    }
+}