Implement on-demand variable fetching

[Kuree]

For generator frameworks where the instances can appear as a local variable, we can actually encoded and make it readable through the debugger. This requires couple changes

1. Local variables or generator variables linking, which implies that the variable table needs a flag which indicates whether it is a instance. The value can be mapped to the instance_id if required.
2. Communication between the runtime and the debugger. VSCode require a variable handle so needs to refactor how it's reading out variable handles.