Refactor control flow in emulator and processor framework

[mnemonikr]

This is a decently sized refactoring that does the following:

• Replaces the emulator ControlFlow enum with the std::ops::ControlFlow enum instead.
• Eliminates the PcodeEmulator trait and renames StandardPcodeEmulator to PcodeEmulator
• Moves Halt control flow from the emulator to the processor framework
• Processors no longer define custom emulators, but instead attach a custom EmulatorHandler that can run both before and after the emulator. Kernel invocations are implemented with this framework now.

There are still some language or other stale references to the old model. For example, both aarch and x86 architectures define emulator.rs as if they are implementing emulators, but instead are now implementing emulator handlers. Leaving this for now.