Migrate to Rust edition 2018 + fix some clippy warnings

[sameer]

Migrated the code to edition to 2018. Changes include:

• try!(abc) --> abc?
• Box<X> --> Box<dyn X>
• No need for extern crate
• use something_from_crate::ABC --> use crate::something_from_crate::ABC
• &String --> &str
• Simplify some struct creations ABC { x: x} --> ABC { x}

Edit: I'm not sure if this necessitates a version bump, but this isn't actually changing any functionality