Consolidate raw representations of rust values

This moves the raw struct layout of closures, vectors, boxes, and strings into a
new `unstable::raw` module. This is meant to be a centralized location to find
information for the layout of these values.

As safe method, `repr`, is provided to convert a rust value to its raw
representation. Unsafe methods to convert back are not provided because they are
rarely used and too numerous to write an implementation for each (not much of a
common pattern).