approx and deepApprox to safely compare numbers

Testing equallity of numbers is a tricky business because of possible round of errors in floating point numbers. For example the following test will unexpectedly fail as the result of the left expression is `0.22000000000000003` and not `0.22`.

``` js
test.equal(2.2 / 10, 0.22);
```

It would be great to have functions like `approx` and `deepApprox` to safely compare numbers and objects/arrays containing numbers against each other. The function should compare whether the difference between the two numbers is at least a predefined factor epsilon smaller than the values itself (I guess there will be some edge cases when one of the values is zero, NaN, or Inf).

首先查看 nodeunit 现有的基于 assert 的比较 API。定义 approx 和 deepApprox 应如何比较数字、对象和数组，包括所述的 epsilon 行为，以及 zero、NaN 和 Infinity 的边界情况。当这些函数及其预期比较行为都已完成规范定义并由测试覆盖时，即视为完成。

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