[swift-evolution] Proposal: Add SequenceType.first

[Brent Royal-Gordon]

> `buffered` is no more problematic than `lazy` is. In fact, calling `buffered` actually doesn't have any side-effects at all (it can avoid fetching the first element until you call `first` on the result of `buffered`).

If `seq` is a single-pass sequence, then `seq.buffered.first` will consume an element from `seq`, even though you only accessed two properties. That's why I call it problematic.

(If calling `buffered` somehow rendered the original sequence unusable—for instance, if we had some way to express that the `BufferedSequence` takes unique ownership of its base—this wouldn't bother me as much.)

-- 
Brent Royal-Gordon
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