Add foreach and foreach_tiled looping constructs

These make it easier to iterate over arbitrary amounts of data
elements; specifically, they automatically handle the "ragged
extra bits" that come up when the number of elements to be
processed isn't evenly divided by programCount.

TODO: documentation

tests/foreach-10.ispc

@@ -0,0 +1,33 @@
+
+export uniform int width() { return programCount; }
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+#define NA 4
+#define NB 8
+#define NC 7
+    uniform int a[NA][NB][NC];
+
+    for (uniform int i = 0; i < NA; ++i)
+        for (uniform int j = 0; j < NB; ++j)
+            for (uniform int k = 0; j < NC; ++j)
+                a[i][j][k] = 0;
+
+    foreach_tiled (i = 0 ... NA, j = 0 ... NB, k = 0 ... NC) {
+        a[i][j][k] += 1;
+    }
+
+    uniform int errs = 0;
+    for (uniform int i = 0; i < NA; ++i)
+        for (uniform int j = 0; j < NB; ++j)
+            for (uniform int k = 0; j < NC; ++j)
+                if (a[i][j][k] != 1) {
+//CO                    print("% % % = %\n", i, j, k, a[i][j][k]);
+                    ++errs;
+                }
+
+    RET[programIndex] = errs;
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 0;
+}