Two small improvements

In `librustc_apfloat/ieee.rs`, use the iterator.[r]find methods to
simplify the code. In `libserialize/json.rs`, make use of the fact
that `Vec.last` on an empty `Vec` returns `None` to simplify the
code to a single match.

src/libserialize/json.rs

@@ -1387,10 +1387,9 @@ impl Stack {
 
     // Used by Parser to test whether the top-most element is an index.
     fn last_is_index(&self) -> bool {
-        if let Some(InternalIndex(_)) = self.stack.last() {
-            true
-        } else {
-            false
+        match self.stack.last() {
+            Some(InternalIndex(_)) => true,
+            _ => false,
         }
     }