refactor VatManager, prepare for VatWorker

[warner]

This starts the process of #1321, refactoring VatManager and the code by which the kernel interacts with vats (deliveries and syscalls).

Redefine VatManager according to the new architecture in docs/vat-worker.md .
The translation duties in the old vatManager.js are split out into
"translators", which are built separately for each vat, but which don't care
how the VatWorker is implemented.

Devices are now defined as files with a named buildRootDeviceNode function,
just like how vats export a buildRootObject function. However device files
no longer support setup at all.

This should pave the way for new kinds of VatWorkers, such as XS in a child
process.

I removed some leftover code in kernel.js that guarded against cross-Realm
function calls (stringifying arguments, copy arrays into new arrays) because
new-SES means we have only one shared Realm everywhere.

The code in kernel.js that builds the metering tools was factored out to a
separate file.

vatManager.js was broken up into a number of separate files, with
src/kernel/vatManager/vatManager.js being the entry point. The
inbound/outbound translation code was moved out to vatTranslator.js,
because it does not depend upon the particular kind of VatWorker we're using.

The portions of kernel.js which handled syscalls on behalf of vats was
moved out to kernelSyscall.js, and made to work in terms of KernelSyscall
objects (which come out of the translator, which is fed with a VatSyscall
object, which comes from the VatManager). Statistics incrementing is the
responsibility of kernelSyscall.js.

The panic() function was made to take the Error object it will be throwing,
to preserve the stack trace when an exception kills the kernel (as opposed to
an assertion failure).

[warner]

I found that so many functions are being called with lots of arguments, so for some of them I replaced the positional list with a single arg named tools, and then do a destructuring assignment to get the individual names. (I'm a python programmer, I like named arguments). If that feels weird or non-idiomatic, let me know, I'm happy to change it back.

Also, the diff will appear slightly smaller if you tell github to hide whitespace changes: many of the test vats/devices were changed to use buildRootObject/buildRootDeviceNode instead of setup(), and that makes it lose a level of indentation.

[FUDCo]

Looking good. Let's pull the trigger!