[Pitch] Remove the single-pass requirement on Sequence

[Jon_Hull]

Relevant (read the part which the forum collapses at the end):

[Pitch] Remove destructive consumption from Sequence

Swift is a language that embraces value semantics. Many common iterators *can* be implemented with value semantics. Just because we can’t implement *all* iterators with value semantics doesn’t mean we should require them to have reference semantics. It just means you can’t *assume* value semantics when working with iterators in generic code unless / until we have a way to specify a value semantics constraint. That’s not necessarily a bad thing especially when it leaves the door open…