Modularize GC allocations in runtime

Currently, GCs only get to see (and modify) 'large' allocations, e.g. allocations of TLAB blocks (via CH::allocate_new_tlab()) or non-TLAB objects (via CH::mem_allocate()). GCs need to own the whole allocation path, including allocations *from* TLABs. For example, Shenandoah needs to allocate one extra word per object, and do some per-object initialization to set up the forwarding pointer.