`init(capacity:)` on RangeReplaceableCollection

[benrimmington]

@jrose

I've been going the other direction when this has come up in the last few months: there is nothing useful you can do from knowing the current capacity, and so it doesn't make any sense to expose it. Just call reserveCapacity .

When an array/dictionary/set is used as an NSCache alternative, you might want to evict old elements when count == capacity.

But this cache limit could be stored elsewhere, and capacity doesn't need to be a protocol requirement.

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