syntax: Add tokens: Option<TokenStream> to Item

This commit adds a new field to the `Item` AST node in libsyntax to optionally
contain the original token stream that the item itself was parsed from. This is
currently `None` everywhere but is intended for use later with procedural
macros.

src/libsyntax/parse/parser.rs

@@ -4653,6 +4653,7 @@ impl<'a> Parser<'a> {
             node: node,
             vis: vis,
             span: span,
+            tokens: None, // TODO: fill this in
         })
     }