Change Annotation to { code, message, span }

Annotations will now have unique codes: E0000 for errors, W0000 for warnings
and H0000 for hints.  They also now have a span, similar to Entries.  If an
annotation is only related to a single position in the file, the Span's start
and end field should be equal.

Author: stasm

spec/fluent.asdl

@@ -31,7 +31,7 @@ module Fluent
           | Junk(str content)
           attributes (annot* annotations, span span)
 
-    annot = Annotation(str name, str message, int pos)
+    annot = Annotation(str code, str message, span span)
     span = Span(int start, int end)
 
     -- Pattern values