Inline utf-8 crate

[nicoburns]

• Ports Remove dependency on the utf-8 crate tendril#68 from the Tendril repo.
• Fixes Stop using the utf-8 crate #730
• Part of Remove remaining dependencies on the utf-8 crate servo#42853

Vendors the small portion of the utf-8 crate that we use and then removes the dependency.

[simonwuelker]

Nice!

tendril actually defines a wrapper around utf8::Incomplete which becomes sort of unnecessary when we vendor utf8. I have a patch for that ready for merging the two together, so I'll submit that as a followup.

[nicoburns]

I have a patch for that ready for merging the two together, so I'll submit that as a followup.

Sounds great! I think there's quite a lot of just "general Rust code improvements" possible in tendril. But I figured it was better split "inline the existing" and "improving the code" into seperate PRs.