Make `TokenStream` less recursive.

Make `TokenStream` less recursive.

`TokenStream` is currently recursive in *two* ways:

- the `TokenTree` variant contains a `ThinTokenStream`, which can
  contain a `TokenStream`;

- the `TokenStream` variant contains a `Vec<TokenStream>`.

The latter is not necessary and causes significant complexity. This
commit replaces it with the simpler `Vec<(TokenTree, IsJoint)>`.

This reduces complexity significantly. In particular, `StreamCursor` is
eliminated, and `Cursor` becomes much simpler, consisting now of just a
`TokenStream` and an index.

The commit also removes the `Extend` impl for `TokenStream`, because it
is only used in tests. (The commit also removes those tests.)

Overall, the commit reduces the number of lines of code by almost 200.