Figure out real FontBuilder API

> We should perhaps create, and cite, an issue to not use Cow in the FontBuilder API?

_Originally posted by @rsheeter in https://github.com/googlefonts/fontations/pull/153#discussion_r1032826261_

The current API for actually constructing a top-level font object is a hack that I put together for the subsetting experiments, and is definitely not what I want it to look like, long-term.

I don't have a coherent long-term design in my head, but I do have some vague ideas:

- with the existence of a 'SfmtTable' trait (e.g. a trait for associating root tables with their tags) then we should generally be able to have an api that's just like, `builder.add(table)`.
- we want to support collections
- we would also like to figure out how we handle the case where we load, modify, and recompile a font; we don't want this to need to generate the compile versions of all tables.
- figure out where the parallelization happens. Probably the uncompiled objects go into the builder, and then when we hit `build` we compile everything in parallel?