Add mac 🖥, tv 📺 & watch ⌚️ support for Carthage

[SimonRice]

This PR builds on my previous contributions and adds cross platform Carthage support to the project.

I've followed the convention used by RxOptional, Actions and other RxSwiftCommunity projects by having a scheme and target per platform. As annoying as it is adding any new source files to all 4 targets, it seems like it's not possible to have a single target and include all of the flavours of RxSwift - however, I'd be more than happy to be corrected.

Finally, I've removed _Pods.xcodeproj as it's a dead symbolic link.

[bobgodwinx]

@simonrice I am testing this locally we'll merge afterwords.

[SimonRice]

@bobgodwinx take all the time you need ⏳

[serluca]

@simonrice would recommend this approach for this pr