window operator should be renamed segment

[Sawtaytoes]

Since RxJS is the one version with different naming to other libraries, there's no reason to keep the window operator using that name. One, it is a namespace conflict in the browser and also highlights differently in most editors even if you're in Node.js.

My suggestion is to rename this to segment. It makes a lot more sense to what it's actually doing: creating observable segments similar to group.

We could easily alias this in v6.7.0, so it's usable today and depreciated in v7.