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-3.ispc

@@ -7,7 +7,7 @@ export void f_f(uniform float RET[], uniform float aFOO[]) {
     int aa;
     uniform int count = 0;
     if (programIndex < 2)
-        count += packed_load_active(a, 0, &aa);
+        count += packed_load_active(a, &aa);
     RET[programIndex] = count;
 }