support statements

[davidchambers]

Currently all > lines must be expressions. This is awkward when one wishes to define a function for use in doctests, as in sanctuary-js/sanctuary#316:

  //. > global.fib = function fib(n) {
  //. .   return n <= 1 ? n : fib(n - 2) + fib(n - 1);
  //. . }
  //. fib
  //.
  //. > S.fromMaybe_(() => fib(30), S.Just(1000000))
  //. 1000000
  //.
  //. > S.fromMaybe_(() => fib(30), S.Nothing)
  //. 832040

It would be nice to be able to use a function declaration (a statement) instead:

  //. > function fib(n) { return n <= 1 ? n : fib(n - 2) + fib(n - 1); }
  //.
  //. > S.fromMaybe_(() => fib(30), S.Just(1000000))
  //. 1000000
  //.
  //. > S.fromMaybe_(() => fib(30), S.Nothing)
  //. 832040