Improve error handling about chimney HOT 2 CLOSED

Rough idea how to approach it: https://twitter.com/pkrzemin/status/877891921674293249

DerivedTransformer would need to produce instance of a type class even when we didn't match the types or didn't find value provider for required field. Definition would need to change to something like:

trait DerivedTransformer[From, To, Modifiers <: HList] {
  type Result = Either[HList, (From, Modifiers) => To]
  def transformer: Result
}

When progressing derivation, we would need to implement something similar to flatMap, but on type-level. We also should decide on semantics (short circuit or capturing all the errors).

In the included tweet there are singleton type literals used, but at the moment I guess we could live with shapeless witnesses.

from chimney.

Decided to go with custom macros as it should give us more flexibility around prioritizing rules, also should simplify internal implementation and generate code that performs better at runtime.

from chimney.