Fix decoding of types conforming to XMLNodeDecoding (CoreOffice#266)

* Fix case when XMLNodeDecoding wants to decode elementOrAttribute, but value is not specified.
Fix error reporting when key not found for element or attribute.

* Add extra tests for unkeyed containers

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Co-authored-by: Joannis Orlandos <[email protected]>

Adds `.noEmptyElements` option to OutputFormatting (CoreOffice#264)

* Adds `.noEmptyElements` option to OutputFormatting

Opts out of empty element shorthand.

* Remove unnecessary `if #available` check

Fix mixed CDATA usage (CoreOffice#261)

Fixes CoreOffice#260

Implement support for encoding DocTypes (CoreOffice#253)

* Implement support for encoding DocTypes

Refine doc comment for `XMLDecodableSequence`.

Fix typo in `CHANGELOG.md`

Bump version to 0.12.0, update `CHANGELOG.md`