Update Rust Compiler to generate 2018 edition code only

The Rust thrift compiler generates a weird mix of 2015/2018 code (i.e. we have dyn) in generated code, but don't use rooted relative module paths). This makes it hard to reason about the generated code.

It's been two years since the 2018 edition was released, and I think it's time for us to break backwards compatibility and move on to only supporting the new edition.