[ContainersPreview] Add DrainableContainer above RangeReplaceableContainer

[lorentey]

We have types that can usefully conform to DrainableContainer but not the full RangeReplaceableContainer; OrderedSet is one obvious example.

This reminded me that we don't have a removeAll(where:) method yet; we'll need one as well as an in-place consuming variant. Ideally these would be implemented by default using SubrangeConsumer, but that would require a Drain that allows partial consumption -- which would be a large complicating factor. 🤔 The easy choice is to define a higher-order, InputSpan-based consume requirement that preserves whatever elements the client leaves in the span, and then have the predicate-based methods forward to that. However, I'm hoping we can do better.

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