After thorough investigation, let's be explicit that panic() does
not (unconditionally) throw but rather normally returns.

Add explicit default value for err arg too.

Also, clean up a couple awaits on resolveToError(), which does not
return a Promise.

initial policy: most recently used 20 vats are kept online

Note that it's important to handle rejection in wasTerminated: in
addition to the error path for the failed execution, there's an error
generated when shutting down the xsnap subprocess.

 - buildKernel:
   - makeVatWarehouse takes over
     - instantiating static, dynamic vats in start()
     - ephemeral.vats
       - vatWarehouse.lookup() replaces ephemeral.vats.has()
     - addVatManager, removeVatManager
     - deliverToVat
   - re-wire notifyTermination
 - makeVatLoader:
   - create methods return Promise<VatManager>
   - translators are passed in
   - no longer uses vatNameToID, queueToExport
   - notifyTermination has been hoisted to its own file
   - clarify source bundle type
 - test vat-target: exhibit stateful behavior

 - replace boolean shortcuts with helper function
 - convert lookup from arrow function
 - note optimization opportunity: replay vats in parallel

 - note makeVatTranslators precondition
 - punt on provideVatKeeper refactor
 - punt on note about comms vat source repeated

We had a TODO to "add a way to remove a vatKeeper from ephemeral in
kernel.js so that we can get rid of a vatKeeper when we evict its
vat"; I tried a `pruneVatKeeper()` method on `kernelKeeper` but it
failed with `resolution of "kp40" is still pending`.

 - provide type for kernel
 - factor out defensiveCopy

Thanks for review, BW.