Mostly change TODOs to FIXMEs and annotate them

But, one change in io to implement a TODO suggestion (using a
const u8)

src/libsyntax/parse/parser.rs

@@ -2139,7 +2139,7 @@ class parser {
           Is it strange for the parser to check this?
           */
           none {
-            self.fatal("class with no ctor");
+            self.fatal("class with no constructor");
           }
         }
     }