Mutually recursive functions

[levjj]

esverify should support mutually recursive functions.

Example:

function isEven(n) {
  requires(Number.isInteger(n) && n >= 0);
  return n === 0 ? true : isOdd(n - 1);
}

function isOdd(n) {
  requires(Number.isInteger(n) && n >= 0);
  return n === 0 ? false : isEven(n - 1);
}