Use Rust 2018 edition

[samford]

Hyper 0.13 uses async but async functions aren't permitted in the 2015 edition of Rust. This migrates Zola to the 2018 edition, so we can continue moving forward with #782.

The main changes to make this work were related to path clarity. Besides that, this allows us to drop some extern crate statements as well. There may be more tidying that can be done but this at least compiles/tests correctly.

[samford]

For whatever reason, CI was running into errors related to the extern crate statement removals in files in the /components folder, though everything worked fine locally for me (Rust 1.39.0, macOS 10.15). Without those removals, it's now fine on CI.

[Keats]

For the extern crate importing macros, you need to import the macros manually where they are used. I don't know why that would wok locally though

[samford]

For the extern crate importing macros, you need to import the macros manually where they are used. I don't know why that would wok locally though

In this latest commit, I've also removed the extern crate statements that were held back due to #[macro_use] and replaced them with use statements in the appropriate files (as described in the Macro changes section of the Edition Guide).

For what it's worth, I hadn't removed the #[macro_use] extern_crate statements in the previous commits, since removing those did cause errors locally. The failing CI tests were simply related to removing extern crate statements in /components. I'm just using a typical install of 1.39.0 using rustup, so I have no idea why my local setup was fine with it but CI wasn't.

[samford]

The components are now using the 2018 edition as well, with extern crate statements removed and use statements added where appropriate. I think this should be good, unless there are any other related changes that should be made.

It turns out that the reason why my local tests were different than CI was because I was mistakenly not using the --all flag when running cargo test, so it wasn't testing the components. This is what happens when you stay up too late working, I suppose.

[Keats]

Ah, that happens a lot to me as well :p
I just merged a pretty big PR, can you rebase by any chance?

[samford]

I rebased this, resolved the conflicts, and made sure it builds/tests fine, so it should be good to go.

[Keats]

Perfect, thanks!