Leicester Square theatre, LondonThe veteran standup deconstructs the science of laughter before scrolling back to his youth and a 1970s brought pungently back to life 'We're here, folks, to laugh", and "See? We're laughing!" There's a lot of this reflexive shtick at the start of Stephen K Amos's touring show: lots of thinking aloud about laughter, and reminding us that's what we're here to do. One routine asks whethe…