Merge ObjectData and Instance together, part 2

When object support was first added to HHVM, a class named "Instance"
was introduced (deriving from ObjectData) to represent instances of user
defined classes. Since then, things have evolved and HPHPc and HPHPi have
been retired, and now there really is no needed to have ObjectData and
Instance be separate classes anymore.

This diff moves all of the functionality from the Instance class to the
ObjectData and removes the Instance class. In the process, I got rid of
a bunch of dead methods and fixed some indentation and other style issues.

hphp/runtime/base/array/array_data.cpp

@@ -136,7 +136,7 @@ void ArrayData::release() {
 // reads
 
 Object ArrayData::toObject() const {
-  return Instance::FromArray(const_cast<ArrayData *>(this));
+  return ObjectData::FromArray(const_cast<ArrayData*>(this));
 }
 
 int ArrayData::compare(const ArrayData *v2) const {