increase succintness of Future handling

[tomholub]

I'll file an issue to create a slightly different convention here for increased clarity.

Originally posted by @tomholub in #400 (comment)

[tomholub]

[tomholub]

Here I was thinking it would be great if the type could be extended, so that instead of:

 messageSender
 .sendMail(mime: encrypted.mimeEncoded)
 .sink(
 receiveCompletion: { [weak self] result in
 guard let error = result.getError() else {
 return
 }

 self?.showAlert(error: error, message: "compose_error".localized)
 },
 receiveValue: { [weak self] in
 self?.handleSuccessfullySentMessage()
 })
 .store(in: &cancellable)

We would have:

 messageSender
 .sendMail(mime: encrypted.mimeEncoded)
 .sinkFuture(
 receiveValue: { [weak self] in
 self?.handleSuccessfullySentMessage()
 },
 receiveError: { [weak self] error in
 self?.showAlert(error: error, message: "compose_error".localized)
 }
 )
 .store(in: &cancellable)

or

 messageSender
 .sendMail(mime: encrypted.mimeEncoded)
 .sink(
 receiveValue: { [weak self] in
 self?.handleSuccessfullySentMessage()
 }
 receiveError: { [weak self] error in
 self?.showAlert(error: error, message: "compose_error".localized)
 },
 )
 .store(in: &cancellable)

Because I imagine we will be seeing a lot of code like this, it would be great if it could be streamlined as above.