Add #[track_caller] annotation to some panicy VM functions

[eminence]

This improves the error messages a bit if the the panics are triggered in these functions

While adding new code into vm.rs, I often got the order of the stack mixed up, and would run into one of the panics like pop_quantity() "Expected quantity to be on the top of the stack"

Without #[track_caller], the line number printed in the error message would be the line of the call to panic!(). But with #[track_caller], the line number now points to the caller of the pop_xyz() function, which is a bit more useful

[sharkdp]

Thanks