This requires Parser to be movable, so we remove the `default`
destructors from Parser and TokenStream, and give them both move
constructors. Since TokenStream only holds a reference to its tokens,
(and it needs to, to avoid copying when given eg a function's contents,)
we add a manual move constructor for Parser which creates a new
TokenStream from the new Parser's tokens, and then manually copies the
old TokenStream's state.
