Add EventLoopFuture utility methods for handling empty collection values

[gwynne]

Provides:

• EventLoopFuture.nonempty(orError:): If the future's Collection-conforming value is empty, fails the future with the provided error.
• EventLoopFuture.nonemptyMap(or:_:): If the future's Collection-conforming value is empty, returns the provided alternate value, otherwise acts like .map(_:).
• EventLoopFuture.nonemptyMap(_:): If the future's Collection-conforming value is empty, returns an empty instance of the RangeReplaceableCollection-conforming result value, otherwise acts like .map(_:).
• EventLoopFuture.nonemptyFlatMapThrowing(or:_:): If the future's Collection-conforming value is empty, returns the provided alternate value, otherwise acts like .flatMapThrowing(_:).
• EventLoopFuture.nonemptyFlatMapThrowing(_:): If the future's Collection-conforming value is empty, returns an empty instance of the RangeReplaceableCollection-conforming result value, otherwise acts like .flatMapThrowing(_:).
• EventLoopFuture.nonemptyFlatMap(or:_:): If the future's Collection-conforming value is empty, returns a future with the provided alternate value, otherwise acts like .flatMap(_:).
• EventLoopFuture.nonemptyFlatMap(orFlat:_:): If the future's Collection-conforming value is empty, returns the provided alternate future, otherwise acts like .flatMap(_:).
• EventLoopFuture.nonemptyFlatMap(_:): If the future's Collection-conforming value is empty, returns a future with an empty instance of the RangeReplaceableCollection-conforming result value, otherwise acts like .flatMap(_:).

Example usage:

return MyModel.query(on: request.db)
    .filter(\.foo == bar)
    .all()
    .nonempty(orError: Abort(.notFound))
    .map { ...

return MyModel.query(on: request.db)
    .filter(\.foo == bar)
    .all()
    .nonemptyMap { models -> [Something] in
        ...

[codecov-commenter]

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[gwynne]

Huh, I'd swear I put the semver label on it... oh well, doing it manually.