Extend sv_dump() to dump SVt_INVLIST

Extend sv_dump() to dump SVt_INVLIST

This changes the previously unused _invlist_dump() function to be called
from sv_dump() to dump inversion list scalars.  The format for regular
SVt_PVs doesn't give human-friendly output for these.

Since these lists are currently not visible outside the Perl core, the
format is documented only in comments in the function itself.