feat(mime): upgrade to mime v0.3

The new mime crate has several benefits:

- Faster formatting
- Easier to use. Most common mime types are now just constants, like
  `mime::TEXT_PLAIN`.
- Proper suffix support.
- Extensible without breaking backwards compatiblity. This means we can
  always add new constants, but before we couldn't add new variants to the
  enums.
- It's now impossible for a `Mime` to contain invalid tokens. Before,
  with the `Ext(String)` variants, it was possible to create an illegal
  mime.

Closes #738

BREAKING CHANGE: Most uses of `mime` will likely break. There is no more
  `mime!` macro, nor a `Mime` constructor, nor `TopLevel` and `SubLevel`
  enums.

  Instead, in most cases, a constant exists that can now be used.

  For less common mime types, they can be created by parsing a string.

src/header/common/link.rs

@@ -905,9 +905,7 @@ mod tests {
     use http::{ServerTransaction, Http1Transaction};
     use bytes::BytesMut;
 
-    use mime::Mime;
-    use mime::TopLevel::Text;
-    use mime::SubLevel::Plain;
+    use mime;
 
     #[test]
     fn test_link() {
@@ -956,7 +954,7 @@ mod tests {
             .push_media_desc(MediaDesc::Screen)
             .set_title("previous chapter")
             .set_title_star("title* unparsed")
-            .set_media_type(Mime(Text, Plain, vec![]));
+            .set_media_type(mime::TEXT_PLAIN);
 
         let link_header = b"<http://example.com/TheBook/chapter2>; \
             rel=\"previous\"; anchor=\"../anchor/example/\"; \
@@ -1015,7 +1013,7 @@ mod tests {
             .push_media_desc(MediaDesc::Screen)
             .set_title("previous chapter")
             .set_title_star("title* unparsed")
-            .set_media_type(Mime(Text, Plain, vec![]));
+            .set_media_type(mime::TEXT_PLAIN);
 
         let link = Link::new(vec![link_value]);