Ensures that destruction will always do the right thing in any context.

- This will be used as a catch-all for slow-but-accurate GPU emulation paths.

The follow-up to e2457418, which
replaces most of the includes in the core header with forward declarations.

This makes it so that if any of the headers the core header was
previously including change, then no one will need to rebuild the bulk
of the core, due to core.h being quite a prevalent inclusion.

This should make turnaround for changes much faster for developers.

- Use a single cached page map.
- Fix calculation of ending page.