* merge.cc (Output_merge_string::do_add_input_section): Count strings

	* merge.cc (Output_merge_string::do_add_input_section): Count strings
	to reserve space in merged_strings vector. Keep total input size
	for stats.
	(Output_merge_string::do_print_merge_stats): Print total input size.
	* merge.h (Output_merge_string): Add input_size_ field.
	* stringpool.cc (Stringpool_template::string_length): Move
	implementations out of Stringpool_template class and place in
	stringpool.h.
	* stringpool.h (string_length): Move out of Stringpool_template.