Overhaul TokenTreeCursor.

- Move it to `rustc_parse`, which is the only crate that uses it. This
  lets us remove all the `pub` markers from it.

- Change `next_ref` and `look_ahead` to `get` and `bump`, which work
  better for the `rustc_parse` uses.

- This requires adding a `TokenStream::get` method, which is simple.

- In `TokenCursor`, we currently duplicate the
  `DelimSpan`/`DelimSpacing`/`Delimiter` from the surrounding
  `TokenTree::Delimited` in the stack. This isn't necessary so long as
  we don't prematurely move past the `Delimited`, and is a small perf
  win on a very hot code path.

- In `parse_token_tree`, we clone the relevant `TokenTree::Delimited`
  instead of constructing an identical one from pieces.

compiler/rustc_parse/src/parser/expr.rs

@@ -8,6 +8,7 @@ use ast::token::IdentIsRaw;
 use ast::{CoroutineKind, ForLoopKind, GenBlockKind, MatchKind, Pat, Path, PathSegment, Recovered};
 use rustc_ast::ptr::P;
 use rustc_ast::token::{self, Delimiter, Token, TokenKind};
+use rustc_ast::tokenstream::TokenTree;
 use rustc_ast::util::case::Case;
 use rustc_ast::util::classify;
 use rustc_ast::util::parser::{AssocOp, ExprPrecedence, Fixity, prec_let_scrutinee_needs_par};
@@ -2393,7 +2394,8 @@ impl<'a> Parser<'a> {
         }
 
         if self.token == TokenKind::Semi
-            && matches!(self.token_cursor.stack.last(), Some((.., Delimiter::Parenthesis)))
+            && let Some(last) = self.token_cursor.stack.last()
+            && let Some(TokenTree::Delimited(_, _, Delimiter::Parenthesis, _)) = last.curr()
             && self.may_recover()
         {
             // It is likely that the closure body is a block but where the