Use `try_from`

[sunfishcode]

As of Rust 1.34, the new TryFrom interfaces are stable in the standard library. We should use these in place of the cast crate where applicable.

For examples of how to do this, see the corresponding PR in Cranelift, although note that Wasmtime uses cast much more extensively.

[kubkon]

That's a great idea, so I'll +1 this!