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.

**UPDATE:** Summary:
The overall plan is to ditch fancy `int` subclasses altogether. But that change would not be backwards-compatible, so for now the changes are:
 - Deprecate `BitwiseInt` in its entirety.
 - Switch `PPField` to use a new, faster `_LBProc` class.
 - Deprecate all the behaviour of `_LBProc` which is beyond that of a standard `int`, and remove any usage of the extended behaviour from Iris.
Innocent, no. Bystander, yes.

`assert_called_once` is not defined by the mock module. Happy to put into a separate PR if you'd prefer.