Integrate vendored "utf-8" crate with tendril types

[simonwuelker]

We vendored utf-8 into tendril. tendril in turn defines utf8_decode::IncompleteUtf8 which is a simple wrapper around utf8::Incomplete that adds a few convenience functions. We can merge these two types together for simplicity.

[simonwuelker]

This enum replaces Option<Result<(), ()>> as a return type from try_complete_offsets :P

[simonwuelker]

This is a breaking change because IncompleteUtf8 now implements Copy - I don't think that's a particularly big deal. We don't need to release a new tendril version for this change.

[nicoburns]

This is a breaking change because IncompleteUtf8 now implements Copy - I don't think that's a particularly big deal. We don't need to release a new tendril version for this change.

I don't think adding a trait impl is a breaking change?

[nicoburns]

Seems reasonable :)

[simonwuelker]

This is a breaking change because IncompleteUtf8 now implements Copy - I don't think that's a particularly big deal. We don't need to release a new tendril version for this change.

I don't think adding a trait impl is a breaking change?

cargo semver-checks says it is in this case: A public type now implements Copy, causing non-move closures to capture it by reference instead of moving it.