Add Exception::CallStack.empty

[straight-shoota]

This is a follow-up on #15002 which explicitly assigns a dummy callstack to RetryLookupWithLiterals for performance reasons.
CallStack.empty is intended to make this use case a bit more ergonomical. It doesn't require allocating a dummy instance with fake data. Instead, it's an explicitly empty callstack. This makes this mechanism easier to re-use in other places (ref #11658 (comment)).

[oprypin]

Worth noting that this changes the behavior. Previously the same instance was used but now a new one is created every time.

[straight-shoota]

Exception::CallStack is a struct, so same instance should not have much significance. Or am I missing something...? 🤔
It's a new Array allocation, though.

[beta-ziliani]

yeah it seems the empty array could be cached (and ask users of .empty to not modify it)

[straight-shoota]

CallStack#@callstack is never modified. It's not even directly accessible from the public API.

I think it should better be a Slice. But unfortunately, changing this a bit tricky because it involves the compiler primitive interpreter_call_stack_unwind. Should still be doable, though.