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

tests/write-same-loc.ispc

@@ -4,12 +4,12 @@ export uniform int width() { return programCount; }
 export void f_fu(uniform float ret[], uniform float aa[], uniform float b) {
     uniform int foo[10];
     for (uniform int i = 0; i < 10; ++i)
-        foo[i] = 10;
+        foo[i] = 10+i;
     int bb = b;
     foo[bb] = 0;
     ret[programIndex] = foo[4] + foo[5];
 }
 
 export void result(uniform float ret[]) {
-    ret[programIndex] = 10;
+    ret[programIndex] = 14;
 }