libstdc++: [_Hashtable] Extend the small size optimization

A number of methods were still not using the small size optimization which
is to prefer an O(N) research to a hash computation as long as N is small.

libstdc++-v3/ChangeLog:

	* include/bits/hashtable.h: Move comment about all equivalent values
	being next to each other in the class documentation header.
	(_M_reinsert_node, _M_merge_unique): Implement small size optimization.
	(_M_find_tr, _M_count_tr, _M_equal_range_tr): Likewise.