Don't require the caller to Box the closure. With the assumption that
the majority of callers *want* the closure to be boxed (i.e. it's not
already boxed), the API becomes easier to use.
For regular builds this is done by adding Rust API that allows
registering a font, and for the web the font is installed into the
browser using JavaScript API.
This is an initial approach to just add this ability. It might make
sense to introduce a syntax in the `.60` file to allow for the
registration of fonts and letting the compiler generate code that
performs this embedding and registration automatically.
