core: io: the read_until function checks bytes not chars, so type should reflect that.

`read_until` is just doing a bytewise comparison. This means the following program prints `xyå12`, not `xy`, which it should if it was actually checking chars.

```rust
fn main() {
    do io::with_str_reader("xyå12") |r| {
        io::println(r.read_until('å', false));
    }
}
```

This patch makes the type of read_until match what it is actually doing.