Use `ascii` crate for string values inside `HeaderValue`, `HeaderName` types

We could remove almost every instance of `unsafe` in our codebase by replacing the use of `String` in `HeaderName` and `HeaderValue` with [`ascii::AsciiString`](https://docs.rs/ascii/1.0.0/ascii/) instead. Implementing this shouldn't be too hard, but code-wise it's a fair few lines that need changing.

This wouldn't change anything in our API signatures either, assuming we don't have any bugs in how we handle ASCII today. @brightly-salty would you be interested in perhaps trying to tackle this?