The call arena doesn't consider nested calls (like, from callbacks). Currently
when a "call" ends, the arena is cleared. But in a callback, if we do that,
the memory for the containing code is no longer valid, even though it's still
executing.
For now, use the existing scope_arena, instead of the call_arena. In the future
we need to track the call-depth, and only reset the call_arena when we're done
with a top-level statement.
Also:
-Properly handle callback errors
-Increase wpt file size
-Merge latest loop.zig from zig-js-runtime.