parser: Add peek(n) method to parser

gcc/rust/ChangeLog:

	* parse/rust-parse.h: New method.

Author: CohenArthur

gcc/rust/parse/rust-parse.h

@@ -725,6 +725,7 @@ template <typename ManagedTokenSource> class Parser
   const ManagedTokenSource &get_token_source () const { return lexer; }
 
   const_TokenPtr peek_current_token () { return lexer.peek_token (0); }
+  const_TokenPtr peek (int n) { return lexer.peek_token (n); }
 
 private:
   // The token source (usually lexer) associated with the parser.