Add foreach_unique iteration construct.

Idea via Ingo Wald / IVL compiler.

tests_errors/foreach-unique-1.ispc

@@ -0,0 +1,12 @@
+// Iteration domain type in "foreach_tiled" loop must be an atomic, pointer, or enum type
+
+struct Point { float x, y; };
+
+uniform int foo(Point p) {
+    uniform int count = 0;
+    foreach_unique (pt in p)
+        ++count;
+    return count;
+}
+
+