wdte: use *Frame

[DeedleFake]

The current implementation uses Frame instead of *Frame, despite it being a linked list. I think the implementation of Scope is good evidence that this isn't necessary. An upside of using *Frame would be that a user could pass nil as the top-level frame.

It might make sense as well to allow the default frame to be overridden. This would allow the user to just use nil but still get access to, for example, std. Maybe something like a DefaultFrame global variable that gets used in the case of a nil frame.