optional_vpart_position should be removed

[scarf005]

The Situation

Cataclysm-BN/src/vpart_position.h

Lines 98 to 117 in c5e21d5

	/**
	* Simple wrapper to forward functions that may return a @ref std::optional
	* to @ref vpart_position. They generally return an empty `optional`, or
	* forward to the same function in `vpart_position`.
	*/
	class optional_vpart_position : public std::optional<vpart_position>
	{
	public:
	optional_vpart_position( std::optional<vpart_position> p ) : std::optional<vpart_position>
	( std::move( p ) ) { }
	std::optional<std::string> get_label() const {
	return has_value() ? value().get_label() : std::nullopt;
	}
	std::optional<vpart_reference> part_with_feature( const std::string &f, bool unbroken ) const;
	std::optional<vpart_reference> part_with_feature( vpart_bitflags f, bool unbroken ) const;
	std::optional<vpart_reference> obstacle_at_part() const;
	std::optional<vpart_reference> part_displayed() const;
	};

Why is it not desirable?

1. It existed because it was before C++17 and used cata::optional. However with Migrate to C++17 #1953 and cata::and_then, it does not have special utility anymore.
2. I'm a newbie in C++, but i think it's less preferable to extend class from STL and forward its methods.

3. Clang-tidy raises false positive over its every single usage.

The solution

1. convert all usages of optional_vpart_position into std::optional<vpart_position>.
2. use cata::and_then for forwarding member functions.