Support for multiple prompt delimiters

[fgmccabe]

Being able to specify exactly which prompt bracket a particular suspension refers to is important for safety. This will help to prevent so-called accidental capture of a suspended computation by the presence of an inner (later) prompt bracket.

In addition, even if it is possible to simulate this by suspending to an inner prompt bracket and then re-suspending if that prompt bracket turns out to be incorrect is unnecessarily inefficient.

[tlively]

@fgmccabe, is this actionable, or can we close it now that we've settled on a design to move forward with?

[fgmccabe]

I would prefer to keep this open for now. There is a connection to the discussion around the barrier instruction