Only add exported and not external functions and types to manifest

examples/math/math.nub

@@ -6,6 +6,11 @@ struct color { r: i32 g: i32 b: i32 a: i32 }
 struct example { a: math::vec2 b: math::vec3 c: math::color }
 
 export func add(a: i32 b: i32): i32
+{
+    return math::add_internal(a b)
+}
+
+func add_internal(a: i32 b: i32): i32
 {
     return a + b
 }