test: move leftover object tests to ds-test

ds-test/simple.c

@@ -1,4 +1,4 @@
-#include <blue/object/string.h>
+#include <blue/ds/string.h>
 
 int main(void)
 {
@@ -6,6 +6,6 @@ int main(void)
 	printf("string object = ");
 	b_object_to_string(string, b_stdout);
 	printf("\n");
-	b_release(string);
+	b_string_unref(string);
 	return 0;
 }

ds-test/unicode-strings.c

@@ -1,5 +1,5 @@
 #include <blue/core/stringstream.h>
-#include <blue/object/string.h>
+#include <blue/ds/string.h>
 #include <stdio.h>
 #include <stdlib.h>