* libguile/strings.c (scm_i_symbol_substring): Return a read-only string
since R5RS requires `symbol->string' to return a read-only string.
Reported by Bill Schottstaedt <bil@ccrma.Stanford.EDU>.
* test-suite/tests/symbols.test: Add `define-module' clause.
(exception:immutable-string): Adjust to current exception.
("symbol->string")["result is an immutable string"]: Use
`pass-if-exception' instead of `expect-fail-exception'.
* NEWS: Update.