Enable Unique!struct unittest

Prevent double delete of Foo by calling release in f().
Cleanup unittests a little.
Enable Unique!class unittest

Call release as a workaround to prevent Bar deletion in g(), leading to
access violation.
Fix Issue 13068 - std.typecons.Unique should disable postblit
Don't delete null reference/pointer
Update Unique docs
Merge pull request #2308 from ntrel/unique-tests

Fix Issue 13068 - std.typecons.Unique should disable postblit