Rust: Replace internal BufReader used in iterator with lightweight Read impl

[leighmcculloch]

The iterator in the Rust uses a BufReader out of convenience as a method for peeking for an EOF, but it is particularly inefficient, especially if the user is already using a BufReader. The only reason we need it is to peek for an EOF. We should find another way to do that with a custom Read impl.

This is a follow up to #102.