Adds a ProcMacro form of syntax extension

This commit adds syntax extension forms matching the types for procedural macros 2.0 (RFC #1566), these still require the usual syntax extension boiler plate, but this is a first step towards proper implementation and should be useful for macros 1.1 stuff too.

Supports both attribute-like and function-like macros.

src/libsyntax/tokenstream.rs

@@ -33,6 +33,7 @@ use parse::lexer::comments::{doc_comment_style, strip_doc_comment_decoration};
 use parse::lexer;
 use parse;
 use parse::token::{self, Token, Lit, Nonterminal};
+use print::pprust;
 
 use std::fmt;
 use std::iter::*;
@@ -781,6 +782,12 @@ impl TokenStream {
     }
 }
 
+impl fmt::Display for TokenStream {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.write_str(&pprust::tts_to_string(&self.to_tts()))
+    }
+}
+
 // FIXME Reimplement this iterator to hold onto a slice iterator for a leaf, getting the
 // next leaf's iterator when the current one is exhausted.
 pub struct Iter<'a> {