add a basic printer into graphviz (http://graphviz.org/) for proofs

- proofs are output into file `proof.dot` if `(get-proof-graph)` is
  in the input
- use `dot -Txlib proof.dot` to see the proof
- use `dot -Tsvg proof.dot` to get a svg file

src/ast/ast_pp_dot.h

@@ -0,0 +1,24 @@
+/*++
+
+Abstract: Pretty-printer for proofs in Graphviz format
+
+--*/
+
+#pragma once
+
+#include <iostream>
+#include "ast_pp.h"
+
+class ast_pp_dot {
+    ast_manager &       m_manager;
+    proof * const       m_pr;
+
+  public:
+    ast_pp_dot(proof *pr, ast_manager &m) : m_manager(m), m_pr(pr) {}
+    ast_pp_dot(proof_ref &e) : m_manager(e.m()), m_pr(e.get()) {}
+
+    std::ostream & pp(std::ostream & out) const;
+    ast_manager & get_manager() const { return m_manager; }
+};
+
+std::ostream &operator<<(std::ostream &out, const ast_pp_dot & p);