adding simplifiers layer

simplifiers layer is a common substrate for global non-incremental and incremental processing.
The first two layers are new, but others are to be ported form tactics.

- bv::slice - rewrites equations to cut-dice-slice bit-vector extractions until they align. It creates opportunities for rewriting portions of bit-vectors to common sub-expressions, including values.
- euf::completion - generalizes the KB simplifcation from asserted formulas to use the E-graph to establish a global and order-independent canonization.

The interface dependent_expr_simplifier is amenable to forming tactics. Plugins for asserted-formulas is also possible but not yet realized.

src/ast/simplifiers/CMakeLists.txt

@@ -0,0 +1,8 @@
+z3_add_component(simplifiers
+  SOURCES
+    euf_completion.cpp
+    bv_slice.cpp
+  COMPONENT_DEPENDENCIES
+    euf
+    rewriter
+)