2.x: Single operators factored out, headers added

[akarnokd]

Single operators factored out and made them extend Single directly. Added missing headers.

[JakeWharton]

When extending types that are Serializable we should probably prevent their serialization altogether with:

private Object writeReplace() {
  throw new UnsupportedOperationException();
}

[akarnokd]

I don't know. Can you give some justification?

[JakeWharton]

Well it's likely to fail anyway since Consumer isn't Serializable, but it makes explicit the fact that this type isn't a type that's meant to be serialized. It's only Serializable as a side-effect of extending another type to save memory.

I just ran over the code and changed my mind because all these instances are internal and while they're exposed in the public API it's only through non-serializable interfaces (like SingleSubscriber in this case). You would have to go far out of your way to even try to serialize these instances and it's not worth wasting the methods for something I'm 99.99% sure will never happen.