Port mime-db to Rust to enhance our mime extension guessing

[yoshuawuyts]

Tracking issue: #274

We should consider porting https://github.com/jshttp/mime-db to Rust in a separate crate so that we can better populate our extension guessing.

[brightly-salty]

After a cursory look at the repository, the functionality seems to stem from a few categorical JSON files describing the Mime types and a few scripts to pull them together into one JSON file to be used by other applications. Is this what you are suggesting for the Rust crate, or something else, more idiomatic?

[Fishrock123]

We probably want a tool which can take the json in those files and compile it into a big match statement programmatically, but not on every compile.

[brightly-salty]

I guess we would need to

1. Use something like surf to request the json file
2. Use a json parsing library (serde?) to parse the json and get required data.
3. Generate mime_const macro calls for each media type using the data
4. Generate a match arm for each media type using the extension and the name of the media type

If this is right, we could just add a function that does all this to the current build.rs and link it to a certain GitHub release of mime-db. The build.rs could write to a file in src/media_type/ that is then used by the other modules. Does this make sense?

[brightly-salty]

It looks like this crate might fulfill our needs: https://crates.io/crates/mime_guess

[yoshuawuyts]

@brightly-salty unfortunately that uses a different mime type than the one we use in http-types. We'll indeed need to manually generate the mime-db crate.

Also I'd probably caution against integrating this into build.rs. Instead using something like a git submodule that we can update and link to is likely better. Like @Fishrock123 points out we don't need to run this on every build.rs step since it can be done statically. Which means all we really need is in-repo tooling to perform the codegen.