Fix package assignment with nested aliased packages

Fix package assignment with nested aliased packages

This commit fixes package assignments like *foo:: = *bar:: when both
foo and bar contain nested stashes that are aliases of each other.

mro_package_moved (actually, its auxiliary routine) need to keep a
list of stashes that have been seen as a separate list from those that
are going to have mro_isa_changed_in called on them. Otherwise, some
stashes will simply not be iterated through.

See the test that this adds and its comments. @ISA = @ISA should never
have any effect visible to Perl (with a capital), but it does in that
test case, prior to this commit.

This also fixes another bug that the test case triggered:
riter was not being reset before the second iteration in
mro_gather_and_rename.

Also, the stashes HV (aka the ‘big list’) now holds refcounts on its
elements, as that makes the code simpler as a result of the changes.