Ensuring position is reported on all de errors

Internally, the serde deserializer implementation uses the existing
error type, renamed as DeserializerError. Now, Deserializer returns a
new type Error that always has a range. At the boundaries of where
deserialization happens, if the location is empty, we fill it with
the parser's current offset.

examples/basic.rs

@@ -27,3 +27,8 @@ fn main() {
     let pretty = rsn::to_string_pretty(&posts);
     println!("Pretty form:\n{pretty}");
 }
+
+#[test]
+fn runs() {
+    main();
+}