Bug 910771 (part 1) - Create js/Tracer.h and gc/Tracer.cpp.  r=terrence.
Bug 910771 (part 2) - Move some stuff from JS::Zone to JS::shadow::Zone, and from JSRuntime to JS::shadow::Runtime.h.  r=terrence.
Bug 910771 (part 3) - Include gc/StoreBuffer.h in gc/Barrier.h.  r=terrence.
Bug 910771 (part 4) - Move all the methods of EncapsulatedValue, HeapValue, RelocatableValue, and HeapSlot from gc/Barrier-inl.h to gc/Barrier.h.  r=terrence.
Bug 910771 (part 5) - Move tons of stuff out of inlines.h/-inl.h files into .h files.  r=terrence.