Project: string_u8 and string_view

ID	Status	Owner	Desc
21_1	DONE	jinkaimori	split basic.hpp to avoid circular inclusion
21_4	DONE	jinkaimori	make behavior of as_int consistent with is_int
21_5	DONE	Da	string_u16 and string_u16_view
21_6	DONE	Da	string_u8 as an alias of string
21_7	DONE	Da	Use string_view<char> to re-impl ==, !=, <, <= of string