A required positional after an optional positional is an error.

As documented...

   * If one positional param is optional, everything to the right must be
   optional

       method foo($a, $b?, $c?)  # legal

       method bar($a, $b?, $c)   # illegal, ambiguous

   Does "->bar(1,2)" mean $a = 1 and $b = 2 or $a = 1, $c = 3?

A required positional after a slurpy technically falls under this, but gets
its own specific error message elsewhere.  This is good because "slurpy
args can only come last" is the information the user needs.

For #108