fix(liveslots): rewrite scanForDeadObjects to avoid retire-without-drop

This rewrites scanForDeadObjects(), which is called during
dispatch.bringOutYourDead to process possiblyDeadSet and
possiblyRetiredSet. The new flow should be easier to review and
understand.

The main behavioral difference is to fix a bug (#9939) in which a vref
that appears in possiblyRetiredSet (because e.g. a weak collection was
deleted, which was using that vref as a key), but which 1: lacks a RAM
pillar (Presence object) and 2: was not dropped in this BOYD (e.g. it
has a vdata pillar), used to be sent to the kernel in a bogus
`syscall.retireImports()` call. Because this vref was not previously
dropped by the vat (syscall.dropImports()), this was a vat-fatal
error.

The new code will only retire such a Presence vref if it was not
reachable by the vat.

The new tests are marked as expected to pass again.

thanks @mhofman and @gibson042 for recommendations

fixes #9939

packages/SwingSet/test/gc/gc-vat.test.js

@@ -160,7 +160,7 @@ test('forward to fake zoe', async t => {
 });
 
 // see #9939
-test.failing('drop without retire', async t => {
+test('drop without retire', async t => {
   let targetID;
   let didBOYD = false;
   // const msgs = ['deliver', 'deliver-result', 'syscall', 'syscall-result'];