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.

src/libsyntax/attr/mod.rs

@@ -472,7 +472,7 @@ impl MetaItem {
                                          Token::from_ast_ident(segment.ident)).into());
             last_pos = segment.ident.span.hi();
         }
-        idents.push(self.node.tokens(self.span));
+        self.node.tokens(self.span).append_to_tree_and_joint_vec(&mut idents);
         TokenStream::new(idents)
     }
 
@@ -529,7 +529,9 @@ impl MetaItemKind {
         match *self {
             MetaItemKind::Word => TokenStream::empty(),
             MetaItemKind::NameValue(ref lit) => {
-                TokenStream::new(vec![TokenTree::Token(span, Token::Eq).into(), lit.tokens()])
+                let mut vec = vec![TokenTree::Token(span, Token::Eq).into()];
+                lit.tokens().append_to_tree_and_joint_vec(&mut vec);
+                TokenStream::new(vec)
             }
             MetaItemKind::List(ref list) => {
                 let mut tokens = Vec::new();
@@ -537,7 +539,7 @@ impl MetaItemKind {
                     if i > 0 {
                         tokens.push(TokenTree::Token(span, Token::Comma).into());
                     }
-                    tokens.push(item.node.tokens());
+                    item.node.tokens().append_to_tree_and_joint_vec(&mut tokens);
                 }
                 TokenTree::Delimited(
                     DelimSpan::from_single(span),