The new mime crate has several benefits:
- Faster formatting
- Easier to use. Most common mime types are now just constants, like
`mime::TEXT_PLAIN`.
- Proper suffix support.
- Extensible without breaking backwards compatiblity. This means we can
always add new constants, but before we couldn't add new variants to the
enums.
- It's now impossible for a `Mime` to contain invalid tokens. Before,
with the `Ext(String)` variants, it was possible to create an illegal
mime.
Closes#738
BREAKING CHANGE: Most uses of `mime` will likely break. There is no more
`mime!` macro, nor a `Mime` constructor, nor `TopLevel` and `SubLevel`
enums.
Instead, in most cases, a constant exists that can now be used.
For less common mime types, they can be created by parsing a string.
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