Move the flush method from Stream to Print

This method originally flushed pending input bytes, which makes sense in
Stream. At some point it was changed to flush output bytes instead, but
it was never moved to Print to reflect this.

Since Stream inherits from Print, this should not really affect any
users of the Stream or Print classes. However to prevent problems with
existing implementations of the Print class that do not provide a
flush() implementation, a default implementation is provided. We should
probably remove this at some point in the future, though.