* Replace hyperx Header and Headers with our own implementation
* Remove utf8_b
* Add RFC 1522 encoder
* Fix most tests
* Throw away old tests
* Header encoding tests
* Fix slicing in the middle of a char
* Content-Disposition after rebase
* Fix the rest of the tests
* Fix useless clone clippy warnings
* Remove Headers::get_raw_mut
* HeaderName::new_from_ascii fallible API
* Tidy up HeaderName::new_from_ascii_str
* HeaderName::new_from_ascii(_str) tests
Fixes https://tools.ietf.org/tools/msglint/ warning about the
Content-Type header being too long.
I looked at many emails I received over time and I couldn't find
any with a boundary as long as ours, so this isn't only justified
by making some tool happy.
