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/properties/longhands/font.mako.rs

@@ -326,7 +326,7 @@ ${helpers.predefined_type(
 
         use app_units::Au;
         use cssparser::{Parser, ToCss};
-        use crate::gecko_bindings::structs::FontFamilyType;
+        use crate::values::computed::font::GenericFontFamily;
         use crate::properties::longhands;
         use std::fmt;
         use std::hash::{Hash, Hasher};
@@ -466,7 +466,7 @@ ${helpers.predefined_type(
                 pub ${name}: longhands::${name}::computed_value::T,
             % endfor
             pub system_font: SystemFont,
-            pub default_font_type: FontFamilyType,
+            pub default_font_type: GenericFontFamily,
         }
 
         impl SystemFont {