state: channel/nick: add Nicks()/Channels() accessors

Hi Alex,

I wanted to retrieve all known nicks of a channel (state.Channel struct) but lookup/nicks are private.
This pull request add Channel.Nicks() returning []*Nick and NicksStr() returning []string. The second one is just for convenience, similar to IsOn/IsOnStr.

Similarly, adding Channels/ChannelsStr() to Nick in order to get channels a nick is on.

Cheers,
StalkR