Don't assume list? means (listof char?).

This is a "hot fix" for commit 68ec385d breaking when used by the
markdown package.

This isn't the most-efficient way to do it, however. Although it fixes
the problem it causes a performance threshold test in markdown lib to
fail.
Faster version of fix.

Note that this FAILS 3 tests, but only with respect to the error
messages being nicely formatted. Ordinarily I wouldn't commit without
either making it pass the tests, or at least changing the tests. But
this is a hot fix.  The message format in the tests is still
desirable, I just don't have time to figure out how to get it
efficiently. Meanwhile I have a user for whom downstream packages are
broken.
Merge pull request #24 from greghendershott/fix-result-to-str

Fix result to str