The I/O Device is implemented as a requestor that responds to two kinds of requests. It does not yet support cancellation.
When sent a request with input #?, it produces the next character read
from the input stream (as a fixnum).
WARNING: It is an error to request a read whilst one is in progress.
When sent a request with input fixnum, it writes that character to the
output stream and produces #?.
WARNING: It is an error to request a write whilst one is in progress.