Remove TreeAndSpacing.

A `TokenStream` contains a `Lrc<Vec<(TokenTree, Spacing)>>`. But this is
not quite right. `Spacing` makes sense for `TokenTree::Token`, but does
not make sense for `TokenTree::Delimited`, because a
`TokenTree::Delimited` cannot be joined with another `TokenTree`.

This commit fixes this problem, by adding `Spacing` to `TokenTree::Token`,
changing `TokenStream` to contain a `Lrc<Vec<TokenTree>>`, and removing the
`TreeAndSpacing` typedef.

The commit removes these two impls:
- `impl From<TokenTree> for TokenStream`
- `impl From<TokenTree> for TreeAndSpacing`

These were useful, but also resulted in code with many `.into()` calls
that was hard to read, particularly for anyone not highly familiar with
the relevant types. This commit makes some other changes to compensate:
- `TokenTree::token()` becomes `TokenTree::token_{alone,joint}()`.
- `TokenStream::token_{alone,joint}()` are added.
- `TokenStream::delimited` is added.

This results in things like this:
```rust
TokenTree::token(token::Semi, stmt.span).into()
```
changing to this:
```rust
TokenStream::token_alone(token::Semi, stmt.span)
```
This makes the type of the result, and its spacing, clearer.

These changes also simplifies `Cursor` and `CursorRef`, because they no longer
need to distinguish between `next` and `next_with_spacing`.

compiler/rustc_builtin_macros/src/concat_idents.rs

@@ -20,14 +20,14 @@ pub fn expand_concat_idents<'cx>(
     for (i, e) in tts.into_trees().enumerate() {
         if i & 1 == 1 {
             match e {
-                TokenTree::Token(Token { kind: token::Comma, .. }) => {}
+                TokenTree::Token(Token { kind: token::Comma, .. }, _) => {}
                 _ => {
                     cx.span_err(sp, "concat_idents! expecting comma");
                     return DummyResult::any(sp);
                 }
             }
         } else {
-            if let TokenTree::Token(token) = e {
+            if let TokenTree::Token(token, _) = e {
                 if let Some((ident, _)) = token.ident() {
                     res_str.push_str(ident.name.as_str());
                     continue;