* Have read_namespace_event borrow the ns buffer

The current situation, where the reader owns the namespace buffer leads to a bad api.
Indeed in the inner loop of the `read_namespaced_event`, the reader, mutably borrowed
cannot be used.

Making the caller own the buffer fixes the situation, while still avoiding extra allocation.

* update tests

* update unit_tests

* update xmlns_reader

* Add issue68 example

* allow unused in issue example

* rust fmt and fix test

* cargo fmt new version

* add read_namespaced_event example and bump version

* fix bench

* update changelog