os: refactor os structure, add Symbol.toPrimitive

Refactor the structure of the os module to use more efficient
module.exports = {} pattern.

Add Symbol.toPrimitive support to os methods that return simple
primitives. This is a minor tweak that makes using these slightly
more friendly when doing things like:

```js
var m = `${os.tmpdir}/foo`
```

PR-URL: https://github.com/nodejs/node/pull/12654


Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Alexey Orlenko <eaglexrlnk@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>