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

tests/shuffle2-7.ispc

@@ -0,0 +1,13 @@
+
+export uniform int width() { return programCount; }
+
+export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
+    int16 aa = aFOO[programIndex]; 
+    int16 bb = aa + programCount;
+    int16 shuf = shuffle(aa, bb, programCount + 1);
+    RET[programIndex] = shuf;
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 2 + programCount;
+}