style: Cleanup generic font-family handling.

To be more similar between Rust and C++. This introduces GenericFontFamily and
exposes that plus FontFamilyNameSyntax to C++, using that where appropriate
instead of plain uint8_t as we were doing.

As a follow-up, as discussed on IRC with Jonathan, we can remove the -moz-fixed
family, and turn it just into an alias of Monospace.

The only non-trivial change is the MatchType changes, but they're ok I think.
The code already assumed at most one CSS generic, and the struct still takes 8
bits. I've verified that the relevant tests are passing (though try is closed).

Differential Revision: https://phabricator.services.mozilla.com/D24272

components/style/values/specified/font.rs

@@ -515,7 +515,8 @@ impl From<LengthPercentage> for FontSize {
 }
 
 /// Specifies a prioritized list of font family names or generic family names.
-#[derive(Clone, Debug, Eq, Hash, PartialEq, ToCss, ToShmem)]
+#[derive(Clone, Debug, Eq, PartialEq, ToCss, ToShmem)]
+#[cfg_attr(feature = "servo", derive(Hash))]
 pub enum FontFamily {
     /// List of `font-family`
     #[css(comma)]