Named handlers extension

[dhil]

This patch implements a semantics for named handlers. It adds the following new types and instructions:

• handler ts*
  • A new reference type for handler names.
• resume_with $ct (on ...) : [ts1* (ref null $ct)] -> [ts2*]
  • where $ct ~~ func [ts1* (ref $handler)] -> [ts2*]
  • and $handler ~~ handler ts2*
  • Operationally, it works like resume, but with the small tweak that it generates and passes a unique name to the continuation.
• suspend_to $handler $tag : [ts1* (ref null $handler)] -> [ts2*]
  • where $handler ~~ handler $ts3
  • and $tag ~~ func [ts1*] -> [ts2*]
  • It is similar to suspend, but rather than suspending to the nearest enclosing handler, it suspend to a particular given handler.

I've added a few tests too.