* Replaced all from ...py23 import * with explicit name imports, or removed completely when possible.
* Replaced tounicode() with tostr()
* Changed all BytesIO ans StringIO imports to from io import ..., replaced all UnicodeIO with StringIO.
* Replaced all unichr() with chr()
* Misc minor tweaks and fixes
The table structure requires that all named instances have the same
record size. Slightly refactored the compilation code to make the
logic easier to understand/maintain.
If a named instance has a PostScript name, the name is now printed as
a comment into the XML dump. Also, unit tests on `fvar` are now
exercising all code paths, both with and without PostScript names.
In the previous table-driven implementation, client code had to
know the internals of the ‘fvar’ structure for correctly adding
variation axes to a font. In the new implementation, clients
do not have to futz around with binary offsets (which makes it
more likely that tools build correct fonts).
