Tidying up Array#is

[DavePearce]

Consider the following:

function f(int[] xs) -> (int r):
  ...

Currently, the constraint generated as a precondition for xs will use Array#is and then separately check all valid elements (inbounds) are integers, and everything else (outbounds) is Void.

In fact, the outbounds check could be folded into Array#is. However, for reasons unknown, attempting to do this caused things to fail.