restXML Protocol Serializers (#394)

* restXML Protocol tests passing

* Fix bug in namespace priority resolution

* Fix unit test

* CR feedback

* Fix handwritten serializer test

* Fix the test, take 2

* Update codegen/src/main/kotlin/software/amazon/smithy/rust/codegen/smithy/protocols/parsers/XmlBindingTraitSerializerGenerator.kt

fix typo

Co-authored-by: John DiSanti <johndisanti@gmail.com>

Co-authored-by: John DiSanti <johndisanti@gmail.com>