This introduces a parallel set of functions to the std.mem.indexOfAny
functions. They simply invert the logic, yielding the first/last index
which is *not* contained in the "values" slice.

Inverting this logic is useful when you are attempting to determine the
initial span which contains only characters in a particular set.

* Document the `indexOfNone` family.

These descriptions are somewhat brief, but the functions themselves are
also simple enough to describe in such a way.