Unify --> ==> ~~>, remove IO, add Callback

[japgolly]

The attempt to satisfy everyone by supporting unsafe callbacks (? => Unit) and using Scalaz's IO has created a division and incompatibility between code. Very bad for community. Already seeing small spots of pain with shared libraries & components.

We need to just have one, single, common way.

The solution:

• A simple, lightweight built-in Callback type. All callbacks are wrapped in it - simple.
• We have 1 operator instead of 3.

Will probably do this earlier rather than later. The longer we wait the worse the change is going to be.
Migration is doing to be a day of pain for everyone, but a worthy one-time price to pay for a lifetime of happiness when everyone's libraries & shared components are compatible.

I'm even considering a quick 0.10.0 to get this over and done with before React 0.14 comes out.

[ngbinh]

💯 had to come back and replace all of our unsafePerformIO with IO version.

[japgolly]

At the same time, should probably also remove variance from React type ctors. It's causing such a frustrating headache having to pepper every new addition to extra with special helper methods to deal with type inference problems.

[japgolly]

Done and on https://github.com/japgolly/scalajs-react/tree/topic/v0.10