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

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

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.