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-18.ispc

@@ -0,0 +1,29 @@
+
+export uniform int width() { return programCount; }
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+#define NA 3
+#define NB 8
+    uniform int a[NA][NB];
+
+    for (uniform int i = 0; i < NA; ++i)
+        for (uniform int j = 0; j < NB; ++j)
+            a[i][j] = 0;
+
+    foreach_tiled (i = 0 ... NA, j = 0 ... NB) {
+        a[i][j] += 1;
+    }
+
+    uniform int errs = 0;
+    for (uniform int i = 0; i < NA; ++i)
+        for (uniform int j = 0; j < NB; ++j)
+            if (a[i][j] != 1) {
+                ++errs;
+            }
+
+    RET[programIndex] = errs;
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 0;
+}