upcxx::optional does not conform to C++17 std::optional interface

[Amir Kamil]

Our implementation of optional diverges from the standard C++17 interface in several places. For instance:

• emplace() does not return a reference to the newly created object (fix forthcoming via PR 475).
• There are several constructor and assignment-operator overloads that we do not implement.
• operator*() and value() are missing const&& overloads.
• There are still some residual optional<T&> specializations (comparison operators, std::hash).
• Comparison operators do not allow comparison between optional<T> and optional<U> or U where U != T.

There may be other diversions as well.

[Amir Kamil]

PR 479 fixes the following:

• Adds operator*() const&& and value() const&& overloads for compilers that have move accessors.
• Removes residual optional<T&> specializations.
• Adds comparisons between optional<T> and optional<U>/U.

The following is still outstanding:

• There are several constructor and assignment-operator overloads that we do not implement.

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[Dan Bonachea]

We don't plan to fix the remaining non-compliances.

Users encountering limitations of our upcxx::optional shim are advised to build using -std=c++17 to activate the std::optional implementation.