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
This does not (yet) upgrade ./rust-toolchain
The warnings:
* dead_code "field is never read"
* redundant_semicolons "unnecessary trailing semicolon"
* non_fmt_panic "panic message is not a string literal, this is no longer accepted in Rust 2021"
* unstable_name_collisions "a method with this name may be added to the standard library in the future"
* legacy_derive_helpers "derive helper attribute is used before it is introduced" https://github.com/rust-lang/rust/issues/79202
