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

tests/shuffle-3.ispc

@@ -0,0 +1,12 @@
+
+export uniform int width() { return programCount; }
+
+export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
+    int8 a = aFOO[programIndex]; 
+    int8 shuf = shuffle(a, 1);
+    RET[programIndex] = shuf;
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 2;
+}