Add UART I/O

[larsbrinkhoff]

Add library for using an UART on targets that have it.

[TG9541]

An abstract TX! and ?RX should do the job. These can even be used for buffered I/O if needed.

[larsbrinkhoff]

Right, something like that. I'm thinking I might as well adopt the classical words EMIT, KEY, and KEY?.

An optional extension on top of this would be to have a target resident interpreter.

[larsbrinkhoff]

If multitasking (#48) is loaded, KEY should PAUSE.