libstdc++: [_Hashtable] Implement several small methods implicitly inline

libstdc++: [_Hashtable] Implement several small methods implicitly inline

Make implementation of 3 simple _Hashtable methods implicitly inline.

Avoid usage of const_iterator abstraction within _Hashtable implementation.

Replace several usages of __node_type* with expected __node_ptr.

libstdc++-v3/ChangeLog:

	* include/bits/hashtable_policy.h
	(_NodeBuilder<>::_S_build): Use __node_ptr.
	(_ReuseOrAllocNode<>): Use __node_ptr in place of __node_type*.
	(_AllocNode<>): Likewise.
	(_Equality<>::_M_equal): Remove const_iterator usages. Only preserved
	to call std::is_permutation in the non-unique key implementation.
	* include/bits/hashtable.h (_Hashtable<>::_M_update_begin()): Capture
	_M_begin() once.
	(_Hashtable<>::_M_bucket_begin(size_type)): Implement implicitly inline.
	(_Hashtable<>::_M_insert_bucket_begin): Likewise.
	(_Hashtable<>::_M_remove_bucket_begin): Likewise.
	(_Hashtable<>::_M_compute_hash_code): Use __node_ptr rather than
	const_iterator.
	(_Hashtable<>::find): Likewise.
	(_Hashtable<>::_M_emplace): Likewise.
	(_Hashtable<>::_M_insert_unique): Likewise.