use Collection instead of Occupiable

[seasonZhu]

I see Occupiable.swift: I can't think of a way to combine these collection types. Suggestions welcomed! I think Swift Collection can instead of Occupiable.

import Foundation

public extension Collection {
    var isNotEmpty: Bool { !isEmpty }
}

So I delete Occupiable.swift, then use Collection+Rx.swift. But I also think developers use this extension in their app with same api, maybe it need optimize this api name.

[rxswiftcommunity[bot]]

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