Implement the notion of a "generated unsafe block"

This way syntax extensions can generate unsafe blocks without worrying about
them generating unnecessary unsafe warnings. Perhaps a special keyword could be
added to be used in macros, but I don't think that's the best solution.

src/libsyntax/ast.rs

@@ -479,7 +479,7 @@ pub struct Field {
 #[deriving(Clone, Eq, Encodable, Decodable, IterBytes)]
 pub enum BlockCheckMode {
     DefaultBlock,
-    UnsafeBlock,
+    UnsafeBlock(/* generated internally */ bool),
 }
 
 #[deriving(Clone, Eq, Encodable, Decodable,IterBytes)]