Change 4 byte bitmap to 32 bit single word

Change 4 byte bitmap to 32 bit single word

I presume that the reason this bitmap was expressed in bytes was that
the macros for dealing with that were already readily available and
familiar, and because it could easily be grown.

However, it's extremely unlikely that we would ever need more bits.
This bit map is for the Posix character classes, and no one is making
more of them.  There is currently one spare bit available, and if we
don't back out of the \s and [:space:] unification, a second will become
available in 5.20 or 5.22.

Using a single word is more efficient, so this changes to use that.
Should we ever need more bits, we can change back.