WIP: instance_eval defaults

This is WIP, just opening to discuss.

First mentioned in https://github.com/orgsync/active_interaction/issues/217, we too are interested in `instance_eval`'d defaults. As a first pass, I'm looking at how to pull any calls to `default` out from inside of filter so as to not require an instance to be passed around to all things that may eventually call `default`. This has some problems, w/ this branch the hash default spec fails, and I don't know what else I'm going to run into.

```
  1) HashInteraction with inputs[:a] returns the correct value for :b
     Failure/Error: expect(result[:b]).to eql(x: {})

       expected: {:x=>{}}
            got: {:x=>nil}

       (compared using eql?)

       Diff:
       @@ -1,2 +1,2 @@
       -:x => {},
       +:x => nil,
     # ./spec/active_interaction/integration/hash_interaction_spec.rb:28:in `block (3 levels) in <top (required)>'
```

I'll comment on some other things inline
Eventually default will take `self` as a parameter in this case (and in the replacement of the attr_accessor here: https://github.com/aaronjensen/active_interaction/blob/instance-eval-defaults/lib/active_interaction/base.rb#L168)