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.
(Last notes seem to have collided ?)
Re: not having BitwiseInt at all, you could argue that the only need for this was the restrictive forms we initially adopted for rules, so it can now all be replaced by "normal" bit masking expresssions (mask constants with "&" "&=" and "|=").