ECPair: replaces ECKey/ECPubKey
ECPair: find network from constants
ECPair: throw if non-boolean compressed flag given

Also adds test for network option
ECPair: use ECPair.curve internally
ECPair: move rng to optional params object
ECPair: throw for invalid network
ECPair: rename *PublicKey to *PublicKeyBuffer
doc: no section headers
Address: remove instantiable object