[0.2.0] allow configuration of nested products/coproducts

[arainko]

So currently ducktape is only able to configure fields/cases of the top-level entity it operates on eg.

case class Person(name: String, address: Address)
case class Address(city: String)

so let's imagine that we want to transform Person into something else but the fields in the Address case class don't quite align. In ducktape 0.1.x to configure fields of the Address field we'd have to do a lot of gymnastics (a separate and explicit Transformer for Address -> DestTyle, an ugly transformation in Field.computed(_.address, _.address.into[DestType]...), generally it is not a good experience.

In ducktape 0.2.x we should be able to just do

Field.computed(_.address.city, _.doWhateverYouNeedToDoHere)

What's more we should also be able to configure fields inside coproduct cases with something like:

Field.computed(_.someCoproduct.at[SomeCoproduct.Case1].field1, _.doSomethingHere)

or even configure cases themselves with

Case.const(_.someCoproduct.at[SomeCoproduct.Case1], SomeCoproduct.Case1(12345))

that'd also mean 0.2.x should deprecate the old way of configuring Cases (but still keep them for sourcecompat until at least 0.3.x)