serialize::base64 could use a Show type

[seanmonstar]

I have a string that when written to a writer, needs to be converted to base64. Currently I have to allocate a new string that is encoded, before passing it to fmt.

I'd imagine some like struct Base64Fmt<'a> { s: &'a str, c: Config}.

[sfackler]

I've been meaning to add {To,From}Base64{Writer,Reader} types, but haven't gotten around to doing it.

[steveklabnik]

This issue has been moved to the rustc-serialize repo: rust-lang-deprecated/rustc-serialize#58