(cleanup) Rename crate and package to content_type.

Snake case crate and package names are preferred over squashedcase ones.

I also cleaned up the `.gitignore` and removed use of deprecated `Vec::get`.

Low hanging fruit I know ;)

Links:
* [Crate naming conventions](https://aturon.github.io/style/naming.html)
* [Implied package naming conventions](https://crates.io)