Add std.array.isSliceOf

`isSliceOf` is a complement for `sameHead` and `sameTail`.
@yebblies 

> I'm guessing the code breakage will mean that when/if scope is implemented then in will stop implying scope.

The problem with `in` and code breakage is when `scope` actually starts meaning something and `in` still means `const scope`, because right now, there's now difference between `const` and `in` in the vast majority of cases, but once `scope` is actually properly defined and implemented, then presumably in at least some of those cases, `in`'s behavior would change due to `scope` changing, and if `scope` wasn't valid in that case or was violated, then the code would break. It could be that it's decided at some point that `in` will just mean `const` in order to avoid that kind of breakage, but it's the fact that `in` means `const scope` and `scope` currently does nothing but will probably do something eventually which makes it so that I almost always tell people to not use `in`. I honestly think that we'd be better off deprecating `in` than continuing to use it when its eventual semantics are unknown. So, at this point, I would never use `in`. That being said, in this particular case, I'd expect that `in` will be valid even once `scope` has been properly defined and implemented, since the arrays do not and cannot escape the function. So, it probably isn't a problem to use it. I still wouldn't though.