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.
> Good point- they also belong in BitwiseInt.

But those functions don't actually exist in `BitwiseInt` - they are merely inherited. So I'm not madly keen on adding them purely for the purpose of issuing deprecation warnings. :unamused: In somewhat hand-wavy terms, it's the inheritance relationship with `SplittableInt` which I'd like to deprecate.

In short, I'm not sure exactly what I'd like to do about this yet. Please feel free to try to convince me of a particular solution!