fix symbol->string to return immutable strings (R6RS §5.10)

[LiberalArtist]

The report says that:

the strings returned by symbol->string … are immutable objects …
An attempt to store a new value into a location referred to by an
immutable object should raise an exception with condition type
&assertion.

[LiberalArtist]

I would have added a test, but I was surprised to see that there already is one:

r6rs/r6rs-test/tests/r6rs/mutable-strings.sls

Lines 17 to 20 in 481ca6b

	(test/unspec-or-exn (string-set! (symbol->string 'immutable)
	0
	#\?)
	&assertion)

If I'm understanding correctly, test/unspec-or-exn does not require that an exception be raised. I suppose this is only a "should"-level requirement, which means "that valid reasons may exist in particular circumstances to ignore a statement, but that the implications must be understood and weighed before choosing a different course." Does the test suite intentionally not test such requirements?