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-4.ispc

@@ -0,0 +1,15 @@
+
+export uniform int width() { return programCount; }
+
+uniform int32 s[programCount];
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    for (uniform int i = 0; i < programCount; ++i)
+        s[i] = -1234;
+    atomic_max_local(&s[programIndex], programIndex);
+    RET[programIndex] = s[programIndex];
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = programIndex;
+}