Make BaseTable a new-style object
Load all OpenType sub-structures lazily

Huge win for subsetter and anyone else who doesn't need the entire
tables.  Subsetting a huge font (eg. NotoSans-Regular.ttf) to a
small set (say, main Cyrillic characters) is something like six
times faster now.  Bulk of time was being spent in blowing up the
GPOS kerning pairs and attach points.  Now we don't load those if
they won't be in the final subset.

Slight slowdown for usecases that need the entire table.