Implement #skip: for negative arguments (or just -1)

[daniels220]

Thanks for taking care of the gotcha in pharo#11150. As a follow-up, it would be nice if Zinc streams could support #skip: with negative arguments—or, if fully-general negative skips are difficult to implement, at least the special case of skipping backward exactly one, as in #back. There are many cases where, due to the available methods in the rest of the Stream API, it is more convenient to over-consume and then backtrack rather than use #peek—for instance #upTo: consumes the passed delimiter, which is often appropriate but not always.