Make configurations more lens-like

[arainko]

Right now if we try to do this:

case class Person(age: Int, name: String)

Person(1, "Joe").into[Person].transform(Field.const(_.age, 23))

we'd be greeted with a pretty cryptic error due to the fact that underneath the covers the plan derived for a Person to Person transformation is actually just an identity (i.e. ducktape doesn't create a new Person from the ground up since that'd be wasteful).

We could actually support that use case if we were to derive a lazy 'alternative' plan when encountering an identity transformation which could then be used when traversing the plan in PlanConfigurer which would basically replace the identity transformation with the fallback plan.