Prepare release.

Merge branch 'pr/n127_steinbitglis'

Fix: scalars returned by the scanner do not have their Start and End …

…properties set.

Fix build failure of portable library.

Prepare version 3.4.0.

Update changelog.

Make YamlDocument and YamlStream serializable.

Fix DuplicateAnchorException when using merge keys do merge mappings …

…that contain aliases.

Implement merging a sequence of aliases.

Fix AnchorNotFoundException when another exception occurs during dese…

…rialization.

Update version number.