Remove the extension traits for Readers/Writers

These methods are all excellent candidates for default methods, so there's no
need to require extra imports of various traits.
auto merge of #10079 : alexcrichton/rust/no-reader-util, r=brson

These methods are all excellent candidates for default methods, so there's no need to require extra imports of various traits. Additionally, this was able to remove all the weird underscores after the method names. Yay!