ECPair: replaces ECKey/ECPubKey

This is a `2.0.0` PR.

Mostly just wanted opinions on this design vs the original `ECKey`/`ECPubKey` design.
I personally prefer it, and the semantics are quite strong so despite the optional constructor argument either way; it does make life a lot simpler.

It also simplifies a fair amount of code IMHO.

Open to comments/discussion. @weilu, @jprichardson?
What would we test?
That options.network is an `Object`?

A better check might be to do what is done in `HDNode` and check that it has `pubKeyHash`?