`is_hashable` is broken in C++20 (`std::hash<T>::result_type` and `::argument_type` were removed)

[stevenwdv]

In C++ 20, e.g. rxu::is_hashable<std::string> unexpectedly returns false because std::hash<T>::result_type and ::argument_type were removed (cppreference, P0619R2). See definition of is_hashable:

RxCpp/Rx/v2/src/rxcpp/rx-util.hpp

Lines 1022 to 1028 in 761b932

	template<typename T>
	struct is_hashable<T,
	typename rxu::types_checked_from<
	typename filtered_hash<T>::result_type,
	typename filtered_hash<T>::argument_type,
	typename rxu::callable_result<filtered_hash<T>, T>::type>::type>
	: std::true_type {};

This means functions like observable<T>::distinct() do not work anymore.