Reduced need to On^2 things

sliced the string called on the inner loop, which cuts down on the
number of ops quite a bit.

Also ran into a bug using some non-ASCII chars.

    else if !(determine_char_uniqeness("öwe老☭")) { fail!("") }

throws an error:

    task '<main>' failed at 'assertion failed:
    self.is_char_boundary(begin) && self.is_char_boundary(end)',
    /build/rust/src/rust-0.9/src/libstd/str.rs:2153

should probably file a bug, b/c that example is copy-pasted from the
docs.