Dependency on abandoned crates that will break in future Rust versions.

[cessen]

When building a project of mine that uses Rouille, I now get this warning:

warning: the following packages contain code that will be rejected by a future version of Rust: buf_redux v0.8.4, multipart v0.18.0

This appears to be due to Rouille's dependency on the multipart crate.

Normally I would file this issue against those two crates, but both appear to be abandoned, with their repositories archived on GitHub. Additionally, someone already filed an issue against multipart, and as expected it's been ignored due to the crate being abandoned.

[cessen]

Oops, duplicate of #271. Somehow missed it when I first scanned the open issues.