cpp/io/basic spanbuf/span

@1@ Gets a referencing the written area if std is set in the open mode, or a  referencing the underlying buffer otherwise.

@2@ Makes the perform I/O on the buffer referenced by. Sets pointers to get area, put area, or both.

Return value
@1@ if std is set in the open mode, or a  that references the whole underlying buffer otherwise.

@2@ (none)