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-3.ispc

@@ -0,0 +1,15 @@
+
+export uniform int width() { return programCount; }
+
+uniform int32 s = 0xff;
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    float a = aFOO[programIndex]; 
+    int32 bits = 0xfff0;
+    float b = atomic_xor_local(&s, bits);
+    RET[programIndex] = s;
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = (programCount & 1) ? 0xff0f : 0xff;
+}