(PUP-1811) The match variables are set for in

We had previous not set the match variables for `in` expressions, but
PUP-1811 changes that. This updates the specification to allow for that
and specifies which value with be used to populate the match variables
for the cases of Array and Hash.
Merge pull request #3 from zaphod42/issue/master/pup-1811-match-variables-for-in

(PUP-1811) The match variables are set for in