[Pitch] Have all adapters in the standard library uniformly expose their `base`

[stephencelis]

@lorentey

(I've been particularly annoyed at base properties on concrete slice types such as Substring.base and ArraySlice.base recently. I've seen little to no legitimate use cases for these so far, outside of the slice/collection implementations themselves .)

FWIW, ArraySlice.base is not public. I even brought this up recently:

How to get the base array from an `ArraySlice`?

Slice and Substring have a convenient base property, which is useful for getting access to the whole collection, but ArraySlice seems to have this functionality omitted. Is this intentional? Is it possible to get the whole array from an ArraySlice?

I've found the base property extremely helpful for "framing" a subsequence in its parent context. E.g. when parsing a substring, access to the base makes it easy to render an error that points to the start/end indices of the substring within the larger string. The same use case could be applied to array slices.

Is there a "better" way to achieve this kind of thing without reaching for the base sequence?