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-5.ispc

@@ -0,0 +1,14 @@
+
+export uniform int width() { return programCount; }
+
+uniform int32 s = 0xbeef;
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    float a = aFOO[programIndex]; 
+    float b = atomic_swap_local(&s, programIndex);
+    RET[programIndex] = reduce_max(b);
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 0xbeef;
+}