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`.

clippy_lints/src/crate_in_macro_def.rs

@@ -110,14 +110,14 @@ fn contains_unhygienic_crate_reference(tts: &TokenStream) -> Option<Span> {
 
 fn is_crate_keyword(tt: &TokenTree) -> Option<Span> {
     if_chain! {
-        if let TokenTree::Token(Token { kind: TokenKind::Ident(symbol, _), span }) = tt;
+        if let TokenTree::Token(Token { kind: TokenKind::Ident(symbol, _), span }, _) = tt;
         if symbol.as_str() == "crate";
         then { Some(*span) } else { None }
     }
 }
 
 fn is_token(tt: &TokenTree, kind: &TokenKind) -> bool {
-    if let TokenTree::Token(Token { kind: other, .. }) = tt {
+    if let TokenTree::Token(Token { kind: other, .. }, _) = tt {
         kind == other
     } else {
         false