[WIP] Iterate on BorrowIteration #85690
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This change allows noncopyable and nonescapable Sequence types, with room for generalizing the Element type to be ~Copyable in the future. The new prototype adds a BorrowingIterator associated type to Sequence This commit adds a BorrowingIteratorProtocol for generalized bulk iteration, a BorrowingIteratorAdapter as an implementation of the new Sequence requirement for existing sequences, and a NeverIterator for new noncopyable/nonescapable sequence types. Conformance to Sequence for Span and InlineArray is also included. Issues: - Availability checking must be off due to the availability requirements on the new Sequence.BorrowingIterator associated type - Several existing sequences needed an explicit Element type alias to compile (note that this was during experimentation, so it may be possible to back out these changes) - Some of the default implementations for generalized sequences simply fatal error - InlineArray is a tricky case, due to its conditionally copyable design. It conforms to Sequence unconditionally, so it needs an unconditional implementation of (the original) Iterator associated type. However, when Sequence can use ~Copyable elements, that implementation won't be workable, since (a) it copies the inline array, and (b) it implements the `next() -> Element?` method, which can't return a noncopyable element.
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Adds more on top of #85294