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

[dabrahams (Dave Abrahams)]

@lorentey

So I'm quietly asking, once again: what are some examples for those "entire classes of algorithms" that will be enabled by the addition of EnumeratedSequence.base ?

Obviously it's not about this one adapter's base. Consider Should there be BidirectionalCollection.dropLast(while:)? - #16 by dabrahams which would be impossible had we not exposed the base of reverse collection iterators.