Abstract away reference counting

[kvark]

The current implementation (#633) uses Arc. The plan (outlined in #288) is to support Rc for single-threaded and Arc for multi-threaded. We could use a common trait similar to this

[kvark]

I did a quick run at it and found that HKT are required. Leaving the status as blocked.

[bvssvni]

I don't think Rc is important now, and would like to test the new architecture before adding more abstractions.

[kvark]

Not happened in pre-ll