fix(xsnap): update XS: new WeakMap design, fixed Promise drops

Update the packages/xsnap/moddable submodule to current public branch, plus
an upcoming xsPromise.c fix.

* current public branch includes a new WeakMap design, which drastically
improves GC speed
* the xsPromise.c fix now correctly drops rejection handlers for resolved
promises, which was probably the cause of #3406 (unexpected retention of
Presences used in `E()` calls, resulting in too few GC actions)

refs #3406  (might even close it)
refs #3118

Author: warner