New stream operations

[Todd Allen <Allen@YALE.ARPA>]

I'd like to suggest the following increases in functionality with
respect to streams:

1.  (open? stream) -> boolean

    Indicates whether or not the stream is open.  This would become true
    upon explicitly closing a stream or upon reading the end-of-file on a
    read-only stream.

2.  (open stream [mode-list]) -> stream
    (maybe-open stream [mode-list]) -> stream

    It should be possible to reopen closed streams.  When fielded by a stream,
    the second argument of the open operation (i.e., the mode list) should be
    optional.