[swift-evolution] [Pitch] Add the sign method to the SignedNumberType protocol.

[Adam Nemecek]

Howdy,
I think that the SignedNumberType should implement a method called sign
that will return -1 for negative numbers, 0 for 0 and 1 for positive
numbers. This is similar to the signum method in e.g. Java and similarly
called methods in other languages.

The implementation is fairly straight forward

extension SignedNumberType {
  var sign: Self {
    if self == 0 {
      return 0
    }
    else if self > 0 {
      return 1
    }
    return -1
  }
}

I was trying to implement is without branching by doing (x > 0) - (x < 0)
but I couldn't get the types right so I'm open to suggestions.
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