MediaContainerType, MediaMIMEType, and nsContentTypeParser

[cpearce]

• Import MediaContainerType and MediaMIMEType classes, which are a dependencies of MediaDecoder.
• Import the test for MediaMIMEType; it was a gtest, but I converted it to be called from cargo test.
• MediaMIMEType depends on nsContentTypeParser. Rather than import Gecko's dependencies there, I imported the Rust "mime" crate, and call that over FFI. It mostly works, but fails on one test. I've filed an issue upstream for that (the author already fixed one issue I pointed out yesterday).