Fix subtle error in caching during kind computation that could cause linear

values to be copied.  Rewrite kind computation so that instead of directly
computing the kind it computes what kinds of values are present in the type,
and then derive kinds based on that. I find this easier to think about.

Fixes #4821.

src/libstd/json.rs

@@ -832,7 +832,7 @@ pub impl Decoder: serialize::Decoder {
     fn read_owned_vec<T>(&self, f: fn(uint) -> T) -> T {
         debug!("read_owned_vec()");
         let len = match *self.peek() {
-            List(list) => list.len(),
+            List(ref list) => list.len(),
             _ => die!(~"not a list"),
         };
         let res = f(len);