XS snapshots don't support (native) Compartments

[dckc]

@dtribble is this on your radar? We've known about it since Sep 2020 but to my knowledge it's not scheduled; I wonder where it belongs in our scheduling.

This blocks #730 , #536

In some ways this belongs on the moddable repo, but our milestone tracking tools don't reach that far...

cc @phoddie @kriskowal @erights

[kriskowal]

Update: Moddable has provided us with an XS version where native compartments support dynamic module loading and can in theory replace our shimmed compartments. The ball is in our court to validate the new version of xsnap and compare it to SES shim, with shared 262 tests #870

Using native compartments will require SES to align with XS on a few minor points #934

We will likely need to move xsnap to endo to cross-test.