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-load-1.ispc

@@ -5,7 +5,7 @@ export void f_f(uniform float RET[], uniform float aFOO[]) {
     uniform unsigned int a[programCount];
     a[programIndex] = aFOO[programIndex];
     unsigned int aa;
-    packed_load_active(a, 0, &aa);
+    packed_load_active(a, &aa);
     RET[programIndex] = aa;
 }