* Removed `CFFContext`
* Added `isCFF2` argument to CFFFontSet.decompile/compile, used from
respective ttLib classes
* Index classes get a `isCFF2` argument in constructor (used for
decompiling); must be True/False if `file` argument is not None;
it is stored as self._isCFF2 to support lazy loading
* Removed `TopDictData` class; reuse same `TopDictIndexCompiler` for
both CFF and CFF2
* `CFFWriter` and all `*Compiler` classes get an `isCFF2` argument;
defaults to the parent compiler's `isCFF2` attribute
* Removed `size` argument from `produceItem` method as unused and
useless (`len(data)` is the same)
* psCharStrings: removed useless ByteCodeBase class
* A reference to the TopDict's VarStoreData is passed down to all
the FontDicts' PrivateDict, so it can be used to get the number of
regions while decompiling blend and vsindex operators
See dicussion:
https://github.com/fonttools/fonttools/pull/968#issuecomment-309920007
SimpleCFF2DEcompiler
CFF2CharString
GlobalSubrsIndex2
SubrsIndex2
CharstringIndex2
Working towards using one set of classes for both CFF2 and CFF data.
