Implement the range interface for `optional`

[akrzemi1]

Implement P3168R2 in Boost.Optional in a C++11-compatible manner.

[typenametea]

Thanks for raising the issue 🙂

I'll be looking into implementing this

[akrzemi1]

Thank you!

[akrzemi1]

The following are criteria for a successful implementation.

• Boost.Optional works for objects (implementation is in here), and is also specialized for lvalue references (implementation is here). The range interface should also work for optional references.
• Unit tests, apart from testing the presence of new members and their type, should also demonstrate that optional objects and optioal references can work with a range-based for-loop.
• Paper P3168r2 shows examples how the new interface of optional interoperates with C++20 ranges. It would be nice to have such tests also. I do not consider them absolutely necessary. But these tests should be conditionally compiled, as otherwise they would break on pre-C++20 compilers.
• The "reference" part of the documentation should reflect the new interface. I mean files 27_ref_optional_synopsis.qbk and 28_ref_optional_semantics.qbk.