Remove ReqSrc and ReqMod

They can no longer be generated. Also remove all the associated
code from the emitter.
Fixed ContNext by writing InitNull rather than Uninit.

Apparently, m_received needs to be InitNull, rather than
Uninit.
Some ObjectData/Variant cleanup and dead code removal

This diff eliminates setOpEqual() and appendOpEqual() from Variant, and
it also removes several methods from ObjectData: o_assign_op(), o_argval(),
setDummy(), init(), cloneDynamic(), o_setPublicWithRef(), o_i_set(),
getRedeclaredParent(), and o_propForIteration().

This diff also makes several methods in ObjectData non-virtual: destruct(),
o_toIterArray(), and o_getDynamicProperties(). To achieve this, some of the
logic from Instance was moved to ObjectData.

Finally, this diff gets rid of "hhvm/externals_stubs.cpp" and
"runtime/eval/eval.cpp".
Fix CodeGenerator::cgNInstanceOf

It assumed the result would be in rax, but it isnt always.
Use the correct register.