Reinstate "Create SVt_INVLIST"

Reinstate "Create SVt_INVLIST"

This reverts commit 49cf1d6641a6dfd301302f616e4f25595dcc65d4, which
reverted e045dbedc7da04e20cc8cfccec8a2e3ccc62cc8b, 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 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.