Implement String structure and update Location/Token to use it

v0/test_log.c

@@ -23,10 +23,10 @@ static void test_log_error(void) {
 static void test_log_on_line(void) {
     Location loc = {
         .filename = "test.c",
-        .line_text = "int main() []",
-        .line_text_length = 13,
+        .line_text = { "int main() []", 13 },
         .line = 1,
-        .column = 12
+        .column_start = 12,
+        .column_end = 13
     };
 
     log_on_line(&loc, 13, "unexpected token");