Implement instance member accessors

[CalebFenton]

These are unimplemented currently because it could easily lead to loss of fidelity. For example, if an instance is modified in another thread, or is modified anywhere outside of the virtualized execution path, which is usually common.

This would need to be an option, disabled by default, and with some warnings around it.
iget-* and iput-* will need to be re-worked, and LocalInstance members will need to be emulated

[strazzere]

+1 right meow pls

[CalebFenton]

I got it in a changeset somewhere ;)