Rollup merge of #34385 - cgswords:tstream, r=nrc

syntax-[breaking-change] cc #31645
(Only breaking because ast::TokenTree is now tokenstream::TokenTree.)

This pull request refactors TokenTrees into their own file as src/libsyntax/tokenstream.rs, moving them out of src/libsyntax/ast.rs, in order to prepare for an accompanying TokenStream implementation (per RFC 1566).

src/libsyntax/fold.rs

@@ -25,6 +25,7 @@ use syntax_pos::Span;
 use codemap::{Spanned, respan};
 use parse::token::{self, keywords};
 use ptr::P;
+use tokenstream::*;
 use util::small_vector::SmallVector;
 use util::move_map::MoveMap;