stdlib updates to take advantage of pointers

The packed_{load,store}_active now functions take a pointer to a
location at which to start loading/storing, rather than an array
base and a uniform index.

Variants of the prefetch functions that take varying pointers 
are now available.

There are now variants of the various atomic functions that take
varying pointers (issue #112).

tests/packed-store-1.ispc

@@ -6,7 +6,7 @@ export void f_f(uniform float RET[], uniform float aFOO[]) {
     uniform int pack[2+programCount];
     for (uniform int i = 0; i < 2+programCount; ++i)
         pack[i] = 0;
-    packed_store_active(pack, 2, a);
+    packed_store_active(&pack[2], a);
     RET[programIndex] = pack[programIndex]; 
 }