Ability to write without flushing

[FeldrinH]

In some cases it would be convenient to write to the SerialDevice without flushing. E.g. if I am generating the data byte-by-byte while writing I don't want to flush for every byte for performance reasons.

Currently it seems this is not possible, because all public write methods flush automatically every time, even if there is plenty of room in the internal write buffer. It would be nice to either change the write methods so they don't flush every time or add new variants that don't flush every time.

[FeldrinH]

My more specific proposal is this: I think the write methods should only flush when the internal buffer is full and if the user needs to flush at the end of some application-specific frame or packet then they should explicitly call flush. That allows the user to take better advantage of the internal buffering.