Implement .clone methods on everything

[masak]

tbd

[masak]

Parenthetically, there should also be a very clear policy about where, as part of the macro expansion mechanism, AST fragments are cloned. (Something like, when macro arguments are passed into the macro, when they are spliced into a quasiquote, and after they are returned from the macro and spliced into the mainline code. Or is that once too much...?)

It might also be a good idea to be clear on exactly why we need Q nodes to be mutable in the first place.