Improve variables view

[nya3jp]

We used to have two scopes of variables, "locals" and "arguments". However, VM somehow returns incorrect results for LocalVariable#isArgument(), so we presented confusing views to users. Since it's not very important to distinguish arguments and local variables, we now include them in the same "locals" scope.

Instead, we introduce the new "fields" scope containing fields of the current object (aka "this").

And finally, we now support inspecting the hierarchy of objects.