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.
Merge pull request #110 from evalEmpire/issue/108

A required positional after an optional positional is an error.