Partly addresses <http://bugs.gnu.org/11197>.
Reported by Klaus Stehle <klaus.stehle@uni-tuebingen.de>.
* module/srfi/srfi-6.scm (open-input-string, open-output-string): New
procedures.
* test-suite/tests/srfi-6.test ("open-input-string")["read-char,
Unicode"]: New test.
("open-output-string")["λ"]: New test.