rename `swarm.key` to `wallet.key`

[rampall]

swarm.key should be renamed to wallet.key to be less confusing and to make it's contents more obvious.

[attila-lendvai]

+1

i was also confused by this.

[0xCandy]

I just find IPFS also has the swarm.key, but it has a totally different use case: the private network connecting each entity uses the same swarm key within the same IPFS network.

Refer to this article: http://www.jurnal.iaii.or.id/index.php/RESTI/article/view/4956