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/test/CMakeLists.txt

@@ -39,6 +39,8 @@ add_executable(test-z3
   doc.cpp
   egraph.cpp
   escaped.cpp
+  euf_bv_plugin.cpp
+  euf_arith_plugin.cpp
   ex.cpp
   expr_rand.cpp
   expr_substitution.cpp