Add support for enums.

tests/enum-1.ispc

@@ -0,0 +1,16 @@
+
+enum Foo {
+    ZERO,
+    ONE,
+    TWO
+};
+
+export uniform int width() { return programCount; }
+
+export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
+    RET[programIndex] = ONE; 
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 1;
+}