Merge pull request #216 from mobile46/master

[FirstOrDefault] Fix bug

[Max] Fix Max on IStructEnumerable.

[Version] bump version to 0.28.0

[Union] fix Reset for UnionEnumerator and RefUnionEnumerator.

[Version] bump version to 0.27.0

[BCL] improve #if for ListLayout and DictionaryLayout.

Should fix net6.0 usage.

[SelectMany] add extensions. (#106)

* [SelectMany] WIP

* [SelectMany] WIP 2

* [SelectMany] Add tests and benchmark.

Bump Nuke.Common from 5.1.1 to 5.1.2

Bumps [Nuke.Common](https://github.com/nuke-build/nuke) from 5.1.1 to 5.1.2.
- [Release notes](https://github.com/nuke-build/nuke/releases)
- [Changelog](https://github.com/nuke-build/nuke/blob/develop/CHANGELOG.md)
- [Commits](nuke-build/nuke@5.1.1...5.1.2)

Signed-off-by: dependabot[bot] <support@github.com>

[Build] fix azure build.

[IEnumerable] add tests.