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_ext/concat.rs

@@ -13,12 +13,13 @@ use syntax::ext::base;
 use syntax::ext::build::AstBuilder;
 use syntax::parse::token;
 use syntax_pos;
+use syntax::tokenstream;
 
 use std::string::String;
 
 pub fn expand_syntax_ext(cx: &mut base::ExtCtxt,
                          sp: syntax_pos::Span,
-                         tts: &[ast::TokenTree])
+                         tts: &[tokenstream::TokenTree])
                          -> Box<base::MacResult+'static> {
     let es = match base::get_exprs_from_tts(cx, sp, tts) {
         Some(e) => e,