Set up a second mirror of guest memory; easily accomplished because we were already using memfd_create / CreateFileMappingW.
This lets us simplify a lot of host code that has to access guest memory that may not be active right now, or might have been mprotect()ed to something weird. Activate is only needed now to run guest code, or when the C# side wants to peer into guest memory for memory domains and such (waterboxhost does not share the mirror address with the C# side).
Waterbox guest code now runs on a stack inside the guest memory space. This removes some potential opportunities for nondeterminism and makes future porting of libco-enabled cores easier.
This replaces the old managed one. The only direct effect of this is to fix some hard to reproduce crashes in bsnes.
In the long run, we'll use this new code to help build more waterbox features.
nattthebear