[R6RS] Two notes on input/output ports

[Michael Sperber]

- We use the "+" naming convention for multiple values elsewhere
  (div+mod).  Rename to `open-file-input/output-port'?

- I assume we also need to add `open-reader/writer-port' above, right?
  It would take a reader and a writer, and return an input/output port
  on both.

-- 
Cheers =8-} Mike
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