Faster access to PPField.lbproc

Recent tests of the fast-FieldsFile code have shown that a substantial fraction of the total file-to-Cube conversion process is taken up constructing instances of `BitwiseInt` for `field.lbproc`.

This work-in-progress PR demonstrates an alternative, compute-on-demand implementation which gets rid of the `BitwiseInt` overhead in the fast-FieldsFile case. Comments welcome.
Actually, a search reveals that BitwiseInt is *only* used for PPField.lbproc.
Plus, it seems that most of the functionality inherited from SplittableInt (I haven't checked it all), like the segment names, is actually completely unused -- even str/repr aren't really that useful or appropriate -- and the extra methods, which *are* used (like "&=") are obviously separate and not even applicable to SplittableInt.
So at a glance I think you can do more or less just what you want with this.  In which case, I'd suggest **removing** the derivation from SplittableInt + making it an independent implementation doing only what is needed.