Rust 2018 syntax and misc updates

[lrazovic]

In this PR i wanted to take advantage of the idioms of Rust 2018, update the dependecies to the latest public version and remove deprecated code.

• crossbeam-channel updated from 0.4 to 0.5.
• rand from 0.7 to 0.8.
• crossbeam-utils from 0.7 to 0.8.
• Removed the extern crate and #[macro_use] syntax, replaced by use where it's needed.
• Replaced std::mem::replace with std::mem::take.
• [In unsafe fn] Replaced the decprecated mem::uninitialized with mem::MaybeUninit::uninit().
• [In unsafe code block] Replaced ptr.offset(offset as isize) with ptr.add(offset). relative clippy lint

Tested with:

• cargo +nightly build --release --all-features --all-targets
• cargo +nightly test --all-features --all-targets
• cargo +nightly bench --all-features --all-targets

I didn't formatted the code with cargo fmt because it would have brought too much changes, if necessary I'm ready to send a new patch.

[highfive]

Thanks for the pull request, and welcome! The Servo team is excited to review your changes, and you should hear from @emilio (or someone else) soon.

[bors-servo]

☔ The latest upstream changes (presumably #284) made this pull request unmergeable. Please resolve the merge conflicts.

[atouchet]

crossbeam-channel and crossbeam-utils were updated in #299.

[stefnotch]

I think this can be closed, unless there are some other improvements in this PR?

[mrobinson]

It looks like the removal of the use of extern crate is still relevant. I can update this PR and land that.