[perl #116096] [BUG] Set.new erroneously flattens arrays in Rakudo

[Christian Bartolomaeus via RT]

Arrays are no longer flattened by Set.new:

> say set(([1, 2], [3, 4])).perl
set([1, 2],[3, 4])

I added a test to S02-types/set.t with the following commit: 
https://github.com/perl6/roast/commit/e8fd9ec55d