Add vhea table

[cmyr]

This is more-or-less a copy/paste of hhea.

[behdad]

Can't you share.

[simoncozens]

Colin and I had a good chat earlier, and we will be refactoring fonttool-rs into two parts: one which is just a pure implementation of the OT spec (probably by moving most of the current code into otspec), and one which is a more user-friendly wrapper on top. One design principle behind the otspec part will be that it does no interpretation of the data, just marshals to and from the on-disk format. I know it's kind of anal, but I suggest that because they're specified individually in the spec, assuming that vhea==hhea is an "interpretation". So I'm quite happy with not sharing here.