Switched extra::json to using rt::io.

Unfortunately, due to rust-lang#8579 and rust-lang#6551, extra::workcache can't be made to
work with the new extra::json.  Either ICEs are cause (which is the
state currently) or there are unconstrained types at the callsites for
extra::work_cache::json_encode.

src/libextra/flatpipes.rs

@@ -471,6 +471,9 @@ pub mod flatteners {
         fn from_writer(w: @Writer) -> Self;
     }
 
+    //No longer works due to json now using rt::io
+    //Update when this module changes to rt::io as well?
+    /* 
     impl FromReader for json::Decoder {
         fn from_reader(r: @Reader) -> json::Decoder {
             match json::from_reader(r) {
@@ -487,6 +490,7 @@ pub mod flatteners {
             json::Encoder(w)
         }
     }
+    */
 
     impl FromReader for ebml::reader::Decoder {
         fn from_reader(r: @Reader) -> ebml::reader::Decoder {