Use Boehm's typed allocations

bdwgc has support for type descriptors, to describe the layout of objects to minimize scanning time. The header is here: https://github.com/ivmai/bdwgc/blob/master/include/gc/gc_typed.h

We should take full advantage of this until we switch to mmtk.