Improved XML serialization

- Use quick_xml and serde for all XML response returned by the S3 API.
- Include tests for all structs used to generate XML
- Remove old manual XML escaping function which was unsafe

src/api/lib.rs

@@ -18,3 +18,4 @@ mod s3_delete;
 pub mod s3_get;
 mod s3_list;
 mod s3_put;
+mod s3_xml;