Add support for parsing quasi-quotes, doesn't do anything useful yet.

src/comp/syntax/ast.rs

@@ -277,6 +277,10 @@ enum mac_ {
     mac_embed_type(@ty),
     mac_embed_block(blk),
     mac_ellipsis,
+    // the span is used by the quoter/anti-quoter ...
+    mac_qq(span /* span of expr */, @expr), // quasi-quote
+    mac_aq(span /* span of quote */, @expr), // anti-quote
+    mac_var(uint),
 }
 
 type lit = spanned<lit_>;