net module qq's

[nyonson]

Just dropping some questions here, maybe a better place than an issue, but didn't want to forget them.

• Is ConnectionExt really an extension? Feels more like just a straight up Connection.
• Does the SendError type need to be exposed in ConnectionWriter::send_message()? I think it's just an implementation detail. The take_errors pattern is pretty cool though, I struggled with what to do there.
• Kinda confused by the OpenWriter type name, is that a convention of some sort?

[rustaceanrob]

• I think the idea was to have an identical ConnectionExt under a futures module. Equivalent would be Connection and AsyncConnection. I like the Ext suffix to signify it is extending a base-struct, but it's more a naming thing.
• There aren't additional error variants yet, so I figured the error should be propagated.
• Just the first thing to come to mind, as the struct will only be alive while the connection is "open". It could be named a number of other things related to "inner", "worker", etc