write-fonts should be able to compile glyf/loca tables

[cmyr]

We should also have top-level Glyf and Loca table structs, although these will be slightly funny: because they're so tightly intertwined I think we should make it so that the only way to create these structs will be to go through a builder.

Currently fontc implements this logic locally, but if anyone else ever wants to write these tables it makes sense to just have a common impl in write-fonts.

[anthrotype]

fixed by #559