Add support for int8/int16 types. Addresses issues #9 and #42.

tests/nested-structs-2.ispc

@@ -16,7 +16,7 @@ export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
     uniform Bar bar;
     for (uniform int i = 0; i < 6; ++i)
         for (uniform int j = 0; j < 18; ++j)
-            bar.foo[i].f[j] = 2.;
+            bar.foo[i].f[j] = 2.+b-5;
 
     bar.foo[5].f[a] = a;
     RET[programIndex] = bar.foo[b].f[a];