style: Move fixed-point font types to Rust

Now that cbindgen and rust support const generics, it seems more simple.

This centralizes all the relevant font constants etc in rust and avoids
conversions when going from rust to C++ and vice versa.

Differential Revision: https://phabricator.services.mozilla.com/D148847

components/derive_common/cg.rs

@@ -143,7 +143,10 @@ pub fn fmap_trait_output(input: &DeriveInput, trait_path: &Path, trait_output: &
                         let ident = &data.ident;
                         GenericArgument::Type(parse_quote!(<#ident as #trait_path>::#trait_output))
                     },
-                    ref arg => panic!("arguments {:?} cannot be mapped yet", arg),
+                    &GenericParam::Const(ref inner) => {
+                        let ident = &inner.ident;
+                        GenericArgument::Const(parse_quote!(#ident))
+                    },
                 })
                 .collect(),
             colon2_token: Default::default(),