Adapt code to use new otlLib builder

Make use of https://github.com/fonttools/fonttools/pull/1926 to clean out custom OT table building code.

Also do a lot of other stuff:

Raise a custom statmake.errors.Error instead of generic Python exceptions.
Enforce all axis names having a default English (United States) entry for the Windows platform, Unicode BMP encoding.
Limit additional locations to axes in Stylespace and not already in font.
Enforce the font getting a location for all axes present in Stylespace.

Closes #26. Closes #13. Closes #14. Closes #17 (otlLib does that for us).

daltonmaag / statmake