Write some tests for swap.

src/test/compile-fail/swap-uninit.rs

@@ -0,0 +1,7 @@
+// error-pattern:Unsatisfied precondition
+
+fn main() {
+    auto x = 3;
+    auto y;
+    x <-> y;
+}

src/test/run-pass/swap-1.rs

@@ -0,0 +1,7 @@
+fn main() {
+    auto x = 3;
+    auto y = 7;
+    x <-> y;
+    assert (x == 7);
+    assert (y == 3);
+}

src/test/run-pass/swap-2.rs

@@ -0,0 +1,14 @@
+fn swap[T](&vec[mutable T] v, int i, int j) {
+    v.(i) <-> v.(j);
+}
+
+fn main() {
+    let vec[mutable int] a = [mutable 0,1,2,3,4,5,6];
+    swap(a, 2, 4);
+    assert(a.(2) == 4);
+    assert(a.(4) == 2);
+    auto n = 42;
+    n <-> a.(0);
+    assert(a.(0) == 42);
+    assert(n == 0);
+}