Improve print_tts by changing tokenstream::Spacing.

`tokenstream::Spacing` appears on all `TokenTree::Token` instances,
both punct and non-punct. Its current usage:
- `Joint` means "can join with the next token *and* that token is a
  punct".
- `Alone` means "cannot join with the next token *or* can join with the
  next token but that token is not a punct".

The fact that `Alone` is used for two different cases is awkward.
This commit augments `tokenstream::Spacing` with a new variant
`JointHidden`, resulting in:
- `Joint` means "can join with the next token *and* that token is a
  punct".
- `JointHidden` means "can join with the next token *and* that token is a
  not a punct".
- `Alone` means "cannot join with the next token".

This *drastically* improves the output of `print_tts`. For example,
this:
```
stringify!(let a: Vec<u32> = vec![];)
```
currently produces this string:
```
let a : Vec < u32 > = vec! [] ;
```
With this PR, it now produces this string:
```
let a: Vec<u32> = vec![] ;
```
(The space after the `]` is because `TokenTree::Delimited` currently
doesn't have spacing information. The subsequent commit fixes this.)

The new `print_tts` doesn't replicate original code perfectly. E.g.
multiple space characters will be condensed into a single space
character. But it's much improved.

`print_tts` still produces the old, uglier output for code produced by
proc macros. Because we have to translate the generated code from
`proc_macro::Spacing` to the more expressive `token::Spacing`, which
results in too much `proc_macro::Along` usage and no
`proc_macro::JointHidden` usage. So `space_between` still exists and
is used by `print_tts` in conjunction with the `Spacing` field.

This change will also help with the removal of `Token::Interpolated`.
Currently interpolated tokens are pretty-printed nicely via AST pretty
printing. `Token::Interpolated` removal will mean they get printed with
`print_tts`. Without this change, that would result in much uglier
output for code produced by decl macro expansions. With this change, AST
pretty printing and `print_tts` produce similar results.

The commit also tweaks the comments on `proc_macro::Spacing`. In
particular, it refers to "compound tokens" rather than "multi-char
operators" because lifetimes aren't operators.

library/proc_macro/src/lib.rs

@@ -925,13 +925,12 @@ impl !Sync for Punct {}
 pub enum Spacing {
     /// A `Punct` token can join with the following token to form a multi-character operator.
     ///
-    /// In token streams constructed using proc macro interfaces `Joint` punctuation tokens can be
-    /// followed by any other tokens. \
-    /// However, in token streams parsed from source code compiler will only set spacing to `Joint`
-    /// in the following cases:
-    /// - A `Punct` is immediately followed by another `Punct` without a whitespace. \
-    ///   E.g. `+` is `Joint` in `+=` and `++`.
-    /// - A single quote `'` is immediately followed by an identifier without a whitespace. \
+    /// In token streams constructed using proc macro interfaces, `Joint` punctuation tokens can be
+    /// followed by any other tokens. However, in token streams parsed from source code, the
+    /// compiler will only set spacing to `Joint` in the following cases.
+    /// - When a `Punct` is immediately followed by another `Punct` without a whitespace. E.g. `+`
+    ///   is `Joint` in `+=` and `++`.
+    /// - When a single quote `'` is immediately followed by an identifier without a whitespace.
     ///   E.g. `'` is `Joint` in `'lifetime`.
     ///
     /// This list may be extended in the future to enable more token combinations.
@@ -939,11 +938,10 @@ pub enum Spacing {
     Joint,
     /// A `Punct` token cannot join with the following token to form a multi-character operator.
     ///
-    /// `Alone` punctuation tokens can be followed by any other tokens. \
-    /// In token streams parsed from source code compiler will set spacing to `Alone` in all cases
-    /// not covered by the conditions for `Joint` above. \
-    /// E.g. `+` is `Alone` in `+ =`, `+ident` and `+()`.
-    /// In particular, token not followed by anything  will also be marked as `Alone`.
+    /// `Alone` punctuation tokens can be followed by any other tokens. In token streams parsed
+    /// from source code, the compiler will set spacing to `Alone` in all cases not covered by the
+    /// conditions for `Joint` above. E.g. `+` is `Alone` in `+ =`, `+ident` and `+()`. In
+    /// particular, tokens not followed by anything will be marked as `Alone`.
     #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
     Alone,
 }
@@ -978,8 +976,8 @@ impl Punct {
     }
 
     /// Returns the spacing of this punctuation character, indicating whether it can be potentially
-    /// combined into a multi-character operator with the following token (`Joint`), or the operator
-    /// has certainly ended (`Alone`).
+    /// combined into a multi-character operator with the following token (`Joint`), or whether the
+    /// operator has definitely ended (`Alone`).
     #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
     pub fn spacing(&self) -> Spacing {
         if self.0.joint { Spacing::Joint } else { Spacing::Alone }