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-6.ispc

@@ -0,0 +1,17 @@
+
+export uniform int width() { return programCount; }
+
+
+uniform int foo(int i);
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    for (uniform int i = 0; i < programCount; ++i)
+        RET[i] = 0;
+
+    foreach (i = -2 ... programCount-2)
+        RET[i+2] += 1234;
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 1234;
+}