Define std::hash specializations for C++17 PMR strings

Define std::hash specializations for C++17 PMR strings

These hash specializations should have been added when the pmr::string
and related typedefs were added.

	* include/std/string (__hash_string_base): New class template defining
	operator() for hashing strings.
	(hash<pmr::string>, hash<pmr::u8string>, hash<pmr::u16string>)
	(hash<pmr::u32string>, hash<pmr::wstring>): Define for C++17.
	* testsuite/21_strings/basic_string/hash/hash.cc: New test.
	* testsuite/21_strings/basic_string/hash/hash_char8_t.cc: New test.

From-SVN: r270116