libstdc++: Remove non-standard public members in std::bitset

libstdc++: Remove non-standard public members in std::bitset

This makes _M_copy_from_ptr, _M_copy_from_string and _M_copy_to_string
private, and declares operator<< and operator>> as friends.

Also remove the historical _M_copy_from_string and _M_copy_to_string
overloads. Those were used before DR 396 was implemented but are
not needed now. There are no tests or docs describing them, so I don't
think we intend to support them as extensions.

libstdc++-v3/ChangeLog:

	* include/std/bitset (_M_copy_from_ptr, _M_copy_from_string)
	(_M_copy_to_string): Change access to private.
	(_M_copy_from_string(const basic_string&, size_t, size_t)):
	Remove.
	(_M_copy_to_string(const basic_string&)): Remove.