Deserializer::bytes_read always returns 0 because read field never updated

[SuperFluffy]

As per the suggestion of @dtolnay and @TyOverby on irc.mozilla#rust, I meant to use Deserializer::bytes_read to ensure that the Deserializer reads to the end of my file, but I noticed that the function always returns 0. Looking through the repository, it looks like Deserializer's read field is only ever used twice: when initializing it to 0 in Deserializer::new and when returning it via bytes_read.

It looks like the field is otherwise not updated at any single time during the deserialization process (as quickly checked by grepping through all files in the repository).