Implement i/o procedures for Scheme base library (io)

- call-with-port
- input-port?
- output-port?
- textual-port?
- binary-port?
- port?
- input-port-open?
- output-port-open?
- current-input-port
- current-output-port
- current-error-port
- close-port
- close-input-port
- close-output-port
- open-input-string
- open-output-string
- get-output-string
- open-input-bytevector
- open-output-bytevector
- get-output-bytevector
- read-char
- peek-char
- read-line
- eof-object?
- eof-object
- char-ready?
- read-string
- read-u8
- peek-u8
- u8-ready?
- read-bytevector
- newline
- write-char
- write-string
- write-u8
- write-bytevector
- flush-output-port

NOTE: Only primitives used to implement these procedures have
undergone testing.
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