Open certain data structures to allow GC walking from roots to all reachable values

This just makes certain `internal` members `public` - sufficient for walking the tree of references in terms of reachable values. Plus adds two methods to - `Api.GetUsedStack` and `Api.GetCurrentEnvironment` - which provide roots for the reference tree.

This is one small part of the ongoing `public`-ization process ultimately aimed at making the VM inspectable. This particular part represents minimal changes necessary for collecting garbage in the space of host-allocated handles.