Reserve do as a keyword

Resolves issue #12157. `do` is hereby reinstated as a keyword; no syntax is
associated with it though. Along the way, a unit test had to be adapted, since
it was using `do` as a method identifier.

Breaking changes:

- Any code using `do` as an identifier will no longer work.

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");
     }
 }