Implement Apple's Hypervisor.Framework platform adapter

[StrikerX3]

The Hypervisor.Framework adapter is currently stubbed under modules/hvf. The code contains a lot of TODOs describing what to implement on each part of the code.

I don't own a Mac. While I could theoretically write the code for the platform, I wouldn't be able to test it.

[StrikerX3]

I got a hold of a Mac for a bit of time and started working on this on the hvf branch. Hypervisor.Framework is considerably different from other platforms:

• Only one virtual machine per process
• Each virtual processor must be created and executed in its own thread
  • The existing instantiation loop won't work with more than one processor
• No way to query the platform's features (as far as I can tell)
• Much lower level than the other platforms -- it gives us direct access to the VMCS!
  • Theoretically this allows all features to be implemented, but it also means things like instruction emulation must be done by virt86 itself

I'll leave this on hold while I study the platform further and focus on HAXM support (#4) for now.