Add scope/call stack awareness to locals / list

[dylandreimerink]

Since #13 and 484f163 we have the tools to determine in which function/scope we are and where we came from. The locals command uses the current scope to look for variables, but there is no reason why we can't also inspect variables in functions to which we have yet to return. To would be nice if we could for example do locals -1 and it will show the locals of our parents scope.

Another idea would be to assign indexes to each scope starting from 0 which is our main program and then increasing, this would require you to first execute callstack/cs to get a index.

Yet another approach might be to be able to "switch" scope. We wouldn't be able to continue execution without going back to the actual scope of the current instruction, but it could be some temporary state to browse around the callstack to inspect the locals for example.