Make Pancake exceptions more intuitive

[IlmariReissumies]

The exception syntax added in #1450 has one particularly confusing property: the variable used to receive the exception payload is not binding. So code like this becomes possible to write:

  exception ExampleException : 1;
  
  fun f() { throw ExampleException 1; }
  
  fun g() {
    var 1 x = 0;
    var 1 y = 0;
    try
      y = f()
    catch ExampleException => x { // this is not a binding occurrence of x! it's the same x as is defined above
      y = x + 1;
    }
    return x + y; // returns 3, where one might expect 2 if the outer x was shadowed
  }

I propose to change the semantics of Call so that the variable in exception handler clauses becomes a binding occurrence.

The wordLang mechanism we target still works like above, so somewhere between pan, crep and loop the compiler can invent a fresh name for storing the exception and wrap it around the Call. I'm not sure where the best place to do this would be. Doing it in pan_to_crep probably means some headache with shapes, and doing it lower down means more languages need to have their semantics changed.

[IlmariReissumies]

This issue, along with the changes already made in #1450, subsumes #1008. The discussion in that issue points out an additional problem with the current state: the oddity that variable in the handler cannot be global, though there's nothing in the concrete syntax to suggest this.