[kerning] Move methods around for clarity
[kerning] Move methods around for clarity

Nothing changes here, just moving around code. My hope is that this
will make the code structure more clear. Public methods of the kern
writer come first, then the private methods in roughly the order
they're called in write(), with utility methods last.

Some method names are changed for consistency.

jamesgk/ufo2ft,googlefonts/ufo2ft,moyogo/ufo2ft,googlei18n/ufo2ft,jamesgk/ufo2fdk