Use safe transmute

The main source of unsafety is use of transmute, so it would be nice to find a safe alternative.

I've looked at the libraries that exist, and the one that looks most promising at the moment is [typic](https://github.com/jswrenn/typic). However, the author of typic, as well as a bunch of other people, are now working on an [RFC](https://github.com/rust-lang/project-safe-transmute/blob/master/rfcs/0000-safe-transmute.md) to add safe transmutes to Rust.

Ideally, safe transmute will be ready soonish, and we can use it. However, it's a big RFC, so realistically, it might be a good idea to integrate typic, with the goal of switching to built-in safe transmute eventually.