various `eof` type signatures are not consistent

[steveklabnik]

$ git grep "fn eof"
src/libnative/io/util.rs:pub fn eof() -> IoError {
src/libserialize/json.rs:    fn eof(&self) -> bool { self.ch.is_none() }
src/libstd/io/fs.rs:    pub fn eof(&self) -> bool {
src/libstd/io/mem.rs:    pub fn eof(&self) -> bool { self.pos >= self.buf.len() }
src/libstd/io/mem.rs:    pub fn eof(&self) -> bool { self.pos >= self.buf.len() }
src/libsyntax/print/pp.rs:pub fn eof(p: &mut Printer) -> io::IoResult<()> { p.pretty_...

One IoError, four bools, and one IoResult.

cc @aturon

[aturon]

I think there aren't any problems here in terms of public, stable APIs, so I'm going to go ahead and close this out. We can address any remaining signatures as part of the normal stabilization process.