Update to use 2021 edition, and update deps
I understand this is a big review, but I tried to change the minimal amount to get it compiling under the new circumstances.
Can you separate out the dependency updates?
Ideally each one separately
I've moved the dependency upgrade to https://github.com/servo/core-foundation-rs/pull/545, so I can probably close this PR for now
:umbrella: The latest upstream changes (presumably 75f424f81a7a7671158e6fcd3fbe30a27f048381) made this pull request unmergeable. Please resolve the merge conflicts.
This is probably duplicated by #704 now.