Replace `Cont[FM, ?]` monad trick with interpreter using `EitherT` and
`Writer`.

Finish cats 0.7.2 update and change `deps` method

- Refactor interpreter: split out handling of `FetchOne`, `FetchMany`
  and `Concurrent`, use `XorT` in `processMany` to reduce nested folds,
  use `Validated` in `processMany` and `processConcurrent` to get
  missing ids/identities or some result.
- (Prematurely ?) optimize `Fetch.join`
- Replaced custom implementations of `map2`, `sequence` and `traverse`
  by using `Applicative[Fetch]`
- ...

I moved some methods around in the `Fetch` object and with the
extraction of the three methods in the interpreter, so the diff looks
much bigger than it actually is.

Refactor and use some more cats features