Add add, sub, mul, and div functions to integers

Arithmetic among integers is supportd. Yey!

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/obj/builtin.rs

@@ -15,7 +15,7 @@ pub static PRINTLN_BUILTIN_FUN: Lazy<NativeFunRef> = Lazy::new(|| NativeFun::new
     {
         read_obj!(let str_obj = return_value);
         let str_obj: &Str = str_obj.as_any().downcast_ref()
-            .ok_or_else(|| Error::ValueError { error: "expected str value".to_string(), value: return_value.clone() })?; 
+            .ok_or_else(|| Error::ValueError { error: "expected str value".to_string(), })?; 
         println!("{}", str_obj.value());
     }
 
@@ -35,7 +35,7 @@ pub static PRINT_BUILTIN_FUN: Lazy<NativeFunRef> = Lazy::new(|| NativeFun::new_o
     {
         read_obj!(let str_obj = return_value);
         let str_obj: &Str = str_obj.as_any().downcast_ref()
-            .ok_or_else(|| Error::ValueError { error: "expected str value".to_string(), value: return_value.clone() })?;
+            .ok_or_else(|| Error::ValueError { error: "expected str value".to_string(), })?;
         print!("{}", str_obj.value());
     }