Enable safe ctor, add example
Document isEmpty and make property
Support transfer to unique base class

Note: Implicit return of a Derived lvalue -> Base is not supported.
Note: We don't need to disable the ref overload because postblit is
disabled now.
Add factory unique() for safe construction with 0 arguments
Clarify release docs
Support assignment from compatible Unique type
Add example test for borrowing Unique by ref
Replace unique() with Unique.create() for clarity

The former does not imply a heap allocation.
Document RefT
Version out Unique.create

Leave this for now in case we get language support for checking
uniqueness.
Merge pull request #2346 from ntrel/unique-update

Fix Issue 13157 -  std.typecons.Unique: Support construction and conversion from compatible types