subscription utility onAccountChange for asks/bids?

[etodanik]

I'd be happy to provide a PR for this, but first I want to know if there's interest. From what I see, the Dex UI ends up accessing the _decoded.asks and _decoded.bids and then set up a listener using connection.onAccountChange() on the public key of the ask/bid accounts.

1. First of all, (and this might belong on the UI repo, but I think maybe some of the contributors here can be knowledgable on that particular choice), why access market._decoded.asks and not market. asksAddress? The latter seems cleaner.
2. Would there be interest in a PR that adds the ability to onBidsChange and onAsksChange?