Better Node.js inspect() support [PR-ready]

[cAttte]

currently, the classes in this library expose a method called inspect(). this is fine, but it's not very useful because the thing that it's modeled after, node:util's inspect(), doesn't know how to use it. so if you plainly console.log(something) it will look very ugly. the way to fix this is by defining another method, [Symbol.for("nodejs.util.inspect.custom")]—see this example for Just:

class Just {
    [Symbol.for("nodejs.util.inspect.custom")](_, opts, inspect) {
        return `Just(${inspect(this.__value, opts)})`;
    }
}

as you can see, this method also harnesses the passed-down inspect function to get the sub-value(s) recursively pretty-printed inside the parentheses, which avoids boring white output and [object Object]. having defined this method, it's as simple as e.g. console.log(Just(new Date)) and you get pretty output for free!

this would only affect Node.js behavior; this symbol is not globally registered in the browser so nothing will happen there at all. i'm down to open a PR so just let me know.