Add shortcuts for at, index

[sapizhak]

@julien-truffaut
Closes #955, #956

[julien-truffaut]

Thanks @sapizhak Could you also add a test to check at works with tuples? My guess it that we need to make I contravariant (i.e. def at[-I, A1])

[sapizhak]

Great idea, I have added some tests to cover all the At and Index summoned by at and index functions (probably every instance is called implicitly now in tests, so if a new instance is added, it would be fine to be called in tests too).

As for contravariant position for I.. We cannot set variance annotations for functions in their type parameters, because variance is a quality of classes, whereas functions declared as def only implement FunctionN trait after desugaring.

Apart from that, Function1 ... FunctionN type parameters are already variance annotated: their arguments are contravariant and result type is invariant. An implicit argument scenario would extend a trait where arguments (including an implicit one) are contravariant as well:

trait ImplicitFunction2[-A, -AA, +B] {
    def apply(a: A)(implicit i: AA): B
  }

So an I should already be contravariant! :))

[julien-truffaut]

As for contravariant position for I.. We cannot set variance annotations for functions in their type parameters, because variance is a quality of classes, whereas functions declared as def only implement FunctionN trait after desugaring.

You are completely right, good point.

Great idea, I have added some tests to cover all the At and Index summoned by at and index functions (probably every instance is called implicitly now in tests, so if a new instance is added, it would be fine to be called in tests too).

Cheers, that's a lot of work but at least we are covered :)
Maybe it will be enough to test all the instances for one optic, e.g. Lens but we can always refactor later on.