Add support for "local" atomics.

Also updated aobench example to use them, which in turn allows using
foreach() and thence a much cleaner implementation.

Issue #58.

tests/local-atomics-varyingptr-1.ispc

@@ -0,0 +1,18 @@
+
+export uniform int width() { return programCount; }
+
+uniform unsigned int32 s[programCount];
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    float a = aFOO[programIndex]; 
+    float b = 0;
+    float delta = 1;
+    if (programIndex < 2)
+        atomic_add_local(&s[programIndex], delta);
+    RET[programIndex] = s[programIndex];
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 0;
+    RET[0] = RET[1] = 1;
+}