This eliminates a lot of necessary string manipulation for something
that preg can just do with the \b assertion.
The change also extracts all the work to prepare ->work_phrase (renamed
->regexp) into a separate function. This is to pave the way for future
efficiency gains, but for now I have not done them, so it is easier to
verify by inspection that this commit does not change behaviour.
Also, some tidy-ups to the filterobject structure, to eliminate some
redundant fields.
2abf8fbf86