Write barrier slowpath for pre/post barriers

[qinsoon]

We currently only have object_reference_write_slow() (which we assume is the subsuming barrier slowpath). We will need object_reference_write_pre_slow() and object_reference_write_post_slow().

[wenyuzhao]

I think instead of having a pre and post slow-path, the real problem is to support N different slow-calls, instead of only one.