Check for negative integer indices when doing indexing-out-of-bounds checks

[robdockins]

Fixes #861

I'd like to add some tests and do some additional checking, but I think this fixes the issue.

Related, however: :check should probably fail when we encounter an error condition, the same way that :prove does. Right now, I think it just accepts crashing test cases as passing.

[brianhuffman]

The new bounds checks all look good to me. With the addition of some tests we'll be good to go.

[robdockins]

Disregard my comment above about :check accepting invalid test arguments. The problem, in fact, was that the test just didn't hit upon cases where the input sequence was all zeros, as specified by the test, so it passes even though negative indices are selected.