auto merge of #12170 : aepsil0n/rust/feature/reserve_do_keyword, r=brson

This resolves issue #12157. Does that do it already or is there something else that needs taking care of?  

As a side note, there seems to be some documentation, in which the old existence of the do keyword is explained. The list of keywords is not up-to-date either. But these are certainly separate issues.

src/libsyntax/parse/token.rs

@@ -492,6 +492,7 @@ declare_special_idents_and_keywords! {
         (53,                         Typeof,     "typeof");
         (54,                         Unsized,    "unsized");
         (55,                         Yield,      "yield");
+        (56,                         Do,         "do");
     }
 }