Add tracing to the Host interface

[Fumuran]

This PR:

• Adds new trace.<trace_id> "decorator" instruction to the assembly
• Adds new on_trace() method to the Host trait to handle trace instructions

TODO:

• Update documentation

[Fumuran]

I'm not sure where should we place the documentation for the trace instruction. I can create another file and place it near the event doc file, but they'll be very similar. Should we merge them to create something like Emitting file with docs for both trace and event?

[bobbinth]

Looks good! Thank you! I left a few comments inline.

I'm not sure where should we place the documentation for the trace instruction. I can create another file and place it near the event doc file, but they'll be very similar. Should we merge them to create something like Emitting file with docs for both trace and event?

Yes, but I'd just use the same Events file and create a "Tracing" section in that file.

Also, looking at this now, I wonder if tracing can replace debug instruction. I think we can get all the same info from both - but there are different pros and cons. Let's create an issue for this.

[bobbinth]

Looks good! Thank you!