io::reader_util should be parameterized over readers

[eholk]

impl reader_util for reader {
    ...
}

should be

impl<T: reader> reader_util for T {
    ...
}

[bblum]

Found during ICFP (#2928).

[Dretch]

Given that ReaderUtil now looks like:

impl<T:Reader> ReaderUtil for T { ... }

Then I propose that this bug is fixed and can be closed.