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-8.ispc

@@ -0,0 +1,23 @@
+
+export uniform int width() { return programCount; }
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    float sum1 = 0, sum2 = 0;
+    foreach (x = 0 ... 10, i = 0 ... 6) {
+        sum1 += aFOO[i];
+    }
+
+    for (uniform int x = 0; x < 10; ++x) {
+        for (uniform int i = 0; i < 6; i += programCount) {
+            int index = i + programIndex;
+            if (index < 6)
+                sum2 += aFOO[index];
+        }
+    }
+
+    RET[programIndex] = sum1 - sum2;
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 0;
+}