Create SVt_INVLIST

This reshuffles the svtype enum to remove the dummy slot created in a
previous commit, and add the new SVt_INVLIST type in its proper order.
It still is unused, but since it is an extension of SVt_PV, it must be
greater than that type's enum value.  Since it can't be upgraded to any
other PV type, it comes right after SVt_PV.

Affected tables in the core are updated.