Basic heap tracing framework + debugger command

Adds an extension command to the debugger which provides
heap tracing functionality, and adds the framework needed to get
heap traces. The heaptrace command can dump the current heap to the
debugger session or it can save a GraphViz specification to a file.

I'm hoping to add a backend which can put the graph in GML format,
so it can be explored interactively. I also hope at some point to
see this integrated into FBIDE if we can do that.

hphp/runtime/base/array/array_data.cpp

@@ -581,5 +581,19 @@ void ArrayData::dump(std::ostream &out) {
   }
 }
 
+void ArrayData::getChildren(std::vector<TypedValue *> &out) {
+  if (isSharedMap()) {
+    SharedMap *sm = static_cast<SharedMap *>(this);
+    sm->getChildren(out);
+    return;
+  }
+  for (ssize_t pos = iter_begin();
+      pos != ArrayData::invalid_index;
+      pos = iter_advance(pos)) {
+    TypedValue *tv = nvGetValueRef(pos);
+    out.push_back(tv);
+  }
+}
+
 ///////////////////////////////////////////////////////////////////////////////
 }