Updated VM-bridges document

[Brian Goetz]

> OK, I see what you’re getting at now.  Yes, this is one of the implementation possibilities.  I was mostly looking to validate the concepts before diving into the representational details.  One key point is that the default case should be able to proceed with no bootstrap; a small set of adaptations handles the most important cases, and avoiding an upcall is probably pretty desirable if we can get away with it.  
> 
> It's an optimization, i prefer the VM to recognize a specific BSM and don't upcall it because its semantics is well known, it has the same effect but it's an implementation detail and not something that need to figure in the VM spec.

It’s an optimization, but not in the way you think.  If the purity of the spec were the only concern, then the approach you lay out would make perfect sense.  But, there are other engineering realities, and taking on the full cost of bootstrap upcalls at this particular place in the VM — when we don’t have to yet — may well be a significant (and not yet necessary) engineering effort.