Port data: url parsing to Rust
This uses the data-url crate. One change in behavior is that data-url is strict about data: URLs having a MIME-type chunk in them. This will be an apparent regression from issue #152, which requested that image data be sniffed if a data: URL doesn't have a MIME-type. However, https://fetch.spec.whatwg.org/#data-urls indeed indicates that data: URLs without a MIME-type are invalid. This removes the test files for issue #152, since they are expected to fail now.