7.5.6 SRFI-6 - Basic String Ports

SRFI-6 defines the procedures open-input-string, open-output-string and get-output-string. These procedures are included in the Guile core, so using this module does not make any difference at the moment. But it is possible that support for SRFI-6 will be factored out of the core library in the future, so using this module does not hurt, after all.