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-11.ispc

@@ -0,0 +1,20 @@
+
+export uniform int width() { return programCount; }
+
+uniform unsigned int32 s = 0;
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    float a = aFOO[programIndex]; 
+    float b = 0;
+    if (programIndex & 1)
+        b = atomic_add_local(&s, programIndex);
+    RET[programIndex] = s;
+}
+
+export void result(uniform float RET[]) {
+    uniform int sum = 0;
+    for (uniform int i = 0; i < programCount; ++i)
+        if (i & 1)
+            sum += i;
+    RET[programIndex] = sum;
+}