Upgrade to Rust 2018, `rustfmt`, `clippy`, `cargo fix`

[sppalkia]

This PR modernizes some of the Rust code by using some of the cargo tooling, and upgrading the Rust edition to 2018. In particular, the PR just follows the following steps:

• https://doc.rust-lang.org/nightly/edition-guide/editions/transitioning-an-existing-project-to-a-new-edition.html
• https://users.rust-lang.org/t/automatically-replace-all-try-with/16873

Unused tests (namely, NdIter) were also removed in preparation for the merge into the master branch.

This pull request also adds cargo clippy and cargo fmt to Travis. Both of these tools are now stabilized.