1.0.19.9: elide runtime calls to %COERCE-CALLABLE-TO-FUN in more cases

1.0.19.9: elide runtime calls to %COERCE-CALLABLE-TO-FUN in more cases

 * Core change: %COERCE-CALLABLE-TO-FUN can now convert to
   GLOBAL-FUNCTION.

 * While at it, refactor the whole "make up a form that returns a
   function to use instead of this lvar or source form" thing for
   clarity.

 * Record slightly crazy OPTIMIZATION possibility.