More efficient (Raw)ObjectId loading

[daboross]

Right now, RawObjectId::from_packed_js_val does a fairly inefficient transition which involves three calls into JS to get three different integer values.

I think it'd be more efficient if we instead exposed a raw [u32; 3] chunk of memory to JavaScript, and had it write the three integers into our array directly. Unfortunately, this requires a mutable UnsafeTypedArray. That existing is tracked at koute/stdweb#360.