rustfmt everything once and for all

[flip1995]

Closes #3460

What's done in this PR:

• Format everything
• Install and run rustfmt on travis
• Update CONTRIBUTING.md to tell contributors about rustfmt (and overdue license change)
• Set error_on_line_overflow in rustfmt.toml

I'm sorry @ everyone who has an open PR... 😞

[flip1995]

I thought about the suggestion #3460 (comment) by @waynr. But having a test with rustfmt that is run with cargo test would require that the contributor has rustfmt installed for cargo test to pass. IMO cargo test shouldn't depend on other tools.

[oli-obk]

Awesome!

[waynr]

@flip1995 that makes sense. I would only point out that there is now an implicit dependency that users install and run rustfmt anyway in order to get their PRs merged which could lead to more abandoned PRs by hit-and-run contributors like myself ;)