Migrate crate to 2018 edition

[phansch]

This migrates the crate to the 2018 edition.

With the 2018 edition enabled, this now produces a new deprecation warning:

warning: use of deprecated item 'std::error::Error::description': use the Display impl or to_string()
  --> src/error/mod.rs:22:26
   |
22 |         f.write_str(self.description())
   |                          ^^^^^^^^^^^
   |

Let me know if I should fix that as well.

[buntine]

Oh, sorry, I somehow totally missed this or forgot to action it. I will take a look tonight and merge it in. Thanks!