Consider adding tracing to the Host interface

[bobbinth]

The Host interface provides several several hooks which could be useful for monitor execution of the VM, however, it may be good to introduce a dedicated "tracing" hook so that event/debug hooks wouldn't need to be overloaded with trancing info.

To do this, we could add the following method to the Host interface:

fn on_trace<S: ProcessState>(&mut self, process: &S, trace_id: u32)

Then, in the assembly, we'd introduce a new instruction:

trace.<tace_id>

This instruction would behave similarly to the emit instruction - but would trigger on_trace() function.

We could also then add an option to ExecutionOption to turn tracing on or off. For example, it could look something like:

pub struct ExecutionOptions {
    max_cycles: u32,
    expected_cycles: u32,
    enable_tracing: bool
}

[bobbinth]

Closed by #1198.