Reinstate "regcomp.c: Move inversion list hdr field to SV hdr"

Reinstate "regcomp.c: Move inversion list hdr field to SV hdr"

This reverts commit 2eb2feb9f4a226d0fe0fd3d66e2ce341296f0072, which
reverted d913fb457b732da4c31d0d1b8c085989a7ecd12d, thus reinstating the
latter commit.  It turns out that the error being chased down was not
due to this commit.

Its original message was:

This moves the final field that can vary from the inversion list data
structure into the header of the SV that contains it.  With this commit,
the body of an inversion list is now const.

The field is converted to a U8, to correspond with the header field in
the SV type that we currently use to hold inversion lists.