Fix building with current rust

By now, #10683 has been fixed, so the temporary can be dropped, but
with the DST changes, we now get &[char, ..5], which doesn't coerce to
&[char]. And since only the latter implements CharEq, we have a problem.
Using as_slice() instead of prefixing the vector with &, we get a
&[char] and all is good.
Merge pull request #37 from dotdash/rustup

Fix building with current rust