Use side metadata specs as default for ObjectModel

[qinsoon]

We should have default side metadata specs for ObjectModel so binding developers could use mmtk-core straight away with all object metadata on the side.

[wks]

It should be possible. The Ruby binding currently puts all metadata except the forwarding pointer on the side.

In today's meeting, we discussed that forwarding pointers should not be implemented as a metadata. We can by default place all metadata on the side, and use the first word of an object (relative to the raw object reference) as the forwarding pointer. See #1030 for more discussions.