Add tests for destructuring locals

Author: marijnh

src/test/compile-fail/let-destruct-refutable.rs

@@ -0,0 +1,11 @@
+// error-pattern:refutable pattern
+
+tag xx {
+    xx(int);
+    yy;
+}
+
+fn main() {
+    let @{x:xx(x), y} = @{x: xx(10), y: 20};
+    assert x + y == 30;
+}

src/test/run-pass/for-destruct.rs

@@ -0,0 +1,5 @@
+fn main() {
+    for {x, y}: {x: int, y: int} in ~[{x: 10, y: 20}, {x: 30, y: 0}] {
+        assert x + y == 30;
+    }
+}

src/test/run-pass/for-each-destruct.rs

@@ -0,0 +1,13 @@
+iter x() -> {x: int, y: int} {
+    let i = 0;
+    while i < 40 {
+        put {x: i, y: 30 - i};
+        i += 10;
+    }
+}
+
+fn main() {
+    for each {x, y}: {x: int, y: int} in x() {
+        assert x + y == 30;
+    }
+}

src/test/run-pass/let-destruct.rs

@@ -0,0 +1,8 @@
+tag xx {
+    xx(int);
+}
+
+fn main() {
+    let @{x:xx(x), y} = @{x: xx(10), y: 20};
+    assert x + y == 30;
+}