Extend spacer with callback events

Callback events allow the client of spacer to
get events during exection. The events include
new lemmas and unfolding.

src/muz/spacer/spacer_callback.cpp

@@ -0,0 +1,38 @@
+/**++
+Copyright (c) 2017 Microsoft Corporation and Matteo Marescotti
+
+Module Name:
+
+    spacer_callback.cpp
+
+Abstract:
+
+    SPACER plugin for handling events
+
+Author:
+
+    Matteo Marescotti
+
+Notes:
+
+--*/
+
+#include "spacer_callback.h"
+#include "muz/spacer/spacer_context.h"
+
+
+namespace spacer {
+
+    void user_callback::new_lemma_eh(expr *lemma, unsigned level) {
+        m_new_lemma_eh(m_state, lemma, level);
+    }
+
+    void user_callback::predecessor_eh() {
+        m_predecessor_eh(m_state);
+    }
+
+    void user_callback::unfold_eh() {
+        m_unfold_eh(m_state);
+    }
+
+}