Rework internal flatten function

http://jsperf.com/new-flatten

~~For lodash speeds we'll need to change `push.apply` to something similar to [this clever logic](https://github.com/lodash/lodash/blob/master/lodash.js/#L1616-1619). Let me know if that's more desirable, I have it [already prepared](https://github.com/megawac/underscore/blob/flatten2/underscore.js#L465-483) and its in the jsperf~~
Yap, also once things go to the side of public API they tend to require more checks/guards that the internal forms don't have to worry about.