Implement invoke

[rachitnigam]

Basic implementation for invoke.

TODO:

• Implement compilation for invoke
• Remove the unimplemented!() macro calls.
  • Punt this to the next PR.

[rachitnigam]

Ready to demo this today!

[rachitnigam]

I feel that generating the once register is unnecessary. The control logic that calls the generated invoke group immediately shuts off the execution once the done signal is high. Furthermore, the control logic also makes sure that the path from the go signal to the done signal has a register in between.

TLDR: Generation can probably just be

group invoke {
  comp.go = 1'd1;
  comp.arg1 = ...;
  invoke[done] = comp.done;
}

[rachitnigam]

This is ready to merge. Waiting on #295.

[sampsyo]

Woohoo! Long live invoke!