libstdc++: Use alias-declarations in bits/hashtable_policy,h

libstdc++: Use alias-declarations in bits/hashtable_policy,h

This file is only for C++11 and later, so replace typedefs with
alias-declarations for clarity. Also remove redundant std::
qualification on size_t, ptrdiff_t etc.

We can also remove the result_type, first_argument_type and
second_argument_type typedefs from the range hashers. We don't need
those types to follow the C++98 adaptable function object protocol.

libstdc++-v3/ChangeLog:

	* include/bits/hashtable_policy.h: Replace typedefs with
	alias-declarations. Remove redundant std:: qualification.
	(_Mod_range_hashing, _Mask_range_hashing): Remove adaptable
	function object typedefs.