Add more test combinators

• Add boolean assert: Alcotest.bool (#33, by @zjhmale)
• Add sorted list assert: Alcotest.slist (#34, by @samoht)
• Add pair assert: Alcotest.pair (#34, by @samoht)
• Add simple assert, built using Pervasive.compare and a pretty-printing
  function: Alcotest.of_pp (#34, by @samoht)