std::trie: optimise insert slightly.

This reduces the number of moves/memcpy's we do, which makes insert
faster, especially in cases of keys with long equal prefixes (the
_low_bits tests):

Before:

    bench_insert_large                ... bench:    553966 ns/iter (+/- 64050)
    bench_insert_large_low_bits       ... bench:   1048151 ns/iter (+/- 92484)
    bench_insert_small                ... bench:    168840 ns/iter (+/- 22410)
    bench_insert_small_low_bits       ... bench:    185069 ns/iter (+/- 38332)

After:

    bench_insert_large                ... bench:    422132 ns/iter (+/- 35112)
    bench_insert_large_low_bits       ... bench:    339083 ns/iter (+/- 34421)
    bench_insert_small                ... bench:    134539 ns/iter (+/- 15254)
    bench_insert_small_low_bits       ... bench:     88775 ns/iter (+/- 5746)

src/libstd/trie.rs

@@ -12,7 +12,7 @@
 
 use prelude::*;
 use uint;
-use util::{swap, replace};
+use util::replace;
 use vec;
 
 // FIXME: #5244: need to manually update the TrieNode constructor
@@ -415,39 +415,41 @@ fn find_mut<'r, T>(child: &'r mut Child<T>, key: uint, idx: uint) -> Option<&'r
 
 fn insert<T>(count: &mut uint, child: &mut Child<T>, key: uint, value: T,
              idx: uint) -> Option<T> {
-    let mut tmp = Nothing;
-    let ret;
-    swap(&mut tmp, child);
+    // we branch twice to avoid having to do the `replace` when we
+    // don't need to; this is much faster, especially for keys that
+    // have long shared prefixes.
+    match *child {
+        Nothing => {
+            *count += 1;
+            *child = External(key, value);
+            return None;
+        }
+        Internal(ref mut x) => {
+            return insert(&mut x.count, &mut x.children[chunk(key, idx)], key, value, idx + 1);
+        }
+        External(stored_key, ref mut stored_value) if stored_key == key => {
+            // swap in the new value and return the old.
+            return Some(replace(stored_value, value));
+        }
+        _ => {}
+    }
 
-    *child = match tmp {
+    // conflict, an external node with differing keys: we have to
+    // split the node, so we need the old value by value; hence we
+    // have to move out of `child`.
+    match replace(child, Nothing) {
         External(stored_key, stored_value) => {
-          if stored_key == key {
-              ret = Some(stored_value);
-              External(stored_key, value)
-          } else {
-              // conflict - split the node
             let mut new = ~TrieNode::new();
             insert(&mut new.count,
                    &mut new.children[chunk(stored_key, idx)],
                    stored_key, stored_value, idx + 1);
-              ret = insert(&mut new.count, &mut new.children[chunk(key, idx)],
+            let ret = insert(&mut new.count, &mut new.children[chunk(key, idx)],
                          key, value, idx + 1);
-              Internal(new)
-          }
-      }
-      Internal(x) => {
-        let mut x = x;
-        ret = insert(&mut x.count, &mut x.children[chunk(key, idx)], key,
-                     value, idx + 1);
-        Internal(x)
-      }
-      Nothing => {
-        *count += 1;
-        ret = None;
-        External(key, value)
-      }
-    };
+            *child = Internal(new);
             return ret;
+        }
+        _ => unreachable!()
+    }
 }
 
 fn remove<T>(count: &mut uint, child: &mut Child<T>, key: uint,