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-7.ispc

@@ -0,0 +1,13 @@
+
+export uniform int width() { return programCount; }
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    float sum = 0;
+    foreach (i = 0 ... 6)
+        sum += aFOO[i];
+    RET[programIndex] = reduce_add(sum);
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 21;
+}