“The Book of Mormon” slipped quietly into town four weeks ago without the hype of an out-of-town tryout, just a lot of anticipation thanks to its creative DNA. Matt Stone and Trey Parker, the envelope-pushing pair behind “South Park,” and Robert Lopez, the composer of “Avenue Q,” delivered this baby. And, oh boy, it’s a real winner. The show is blissfully original, irreverent, outspoken and hilarious. And all that’s tucked inside good — no, great — old-fashioned musical. The plot follows two young Mormon missionaries — Elder Price, an ace; Elder Cunningham, a loser — from Utah to Uganda, where they seek to recruit new church members. All of them end up transformed. But that change, and the acknowledgment that it’s okay to believe in what you want, doesn’t come easily. Price and Cunningham find their Mormon faith and American can-do spirit tested to the limit after they run into curious mythologies and Africans brutalized by AIDS, poverty and a sadistic warlord.