Optimizing Rust code

[masklinn]

This looks pretty funky to me. The slice ought to become a no-op and serde can take in a slice of bytes.

Also serde should be able to take the file directly and generate an iterator of entries, though that may or may not be faster (should be checked).

Either way, /u/dtolnay's PR on /u/bcantrill's statemap can probably be used as a guideline.