Switch to Rust 2018 syntax

Rust 2018 has a lot of new syntax items that make the code easier to read. This would be a massive improvement over 1.29, like Non-Lexical Lifetimes and module improvement, like dropping extern crate.

1.29.0 is also 5 month old, and some features start to become slowly deprecated, meaning the longer Ion wait, the more it will have to catch up.

Since cargo offers cargo fix, it should not be tedious work.