Add ACursor with history to codec

[note]

My motivation here was to have a history of operations so for deeply nested ADTs we can have descriptive errors. You can see my driver for that, and an example of usage, here: https://github.com/note/dhallj-magnolia/pull/9/files

This change is inspired heavily on circe's design but I sticked only to things I truly needed in my case.

Of course, I can imagine you don't want such change at all or you prefer to introduce it later after gathering more use cases. If that's the case I can use my own Decoder for my project

[travisbrown]

Sorry to have missed this, and thanks! It looks reasonable to me. We're still planning to revisit the naming in Circe, and might want to change it here eventually, but I think we can go ahead and add this as-is.