Added some vm runtime objects.
Added start of refactored rooting code.
Added scanning/update comments to roots and changed RootKindTrait to be a fuller-fledged RootTraits struct.
Renamed gc/roots.hpp to gc/stack.hpp.  Changed type names to reflect.
Fixed up commenting on stack.hpp
Renamed stack.hpp to local.hpp.  Renamed Stack* types to Local*.  Stack is a common name.
Added top-level GC templates, and specialized for SlabThing pointers.
Added traits implementations for VM::Module.
Unified allocation traits into one structure.