LowPriorityImplicits

Note that much of the implementation of anyValEncoder/anyValDecoder is a workaround for: https://github.com/lampepfl/dotty/issues/12179#issuecomment-826294510

Originally, the idea was to simply pass the self in LowPriorityImplicits directly into the macro that creates the AnyValEncoders. That way, the implementation would be as simple as:

 trait LowPriorityImplicits { self: EncodingDsl =>
   implicit inline def anyValEncoder[Cls <: AnyVal]: Encoder[Cls] =
     new MappedEncoderMaker[Encoder, Cls](self)
 }

Then, the MappedEncoderMaker could just internally call self.mappedEncoder(mapped, encoder) (where this self is the one that is passed in from the LowPriorityImplicits).

Unfortunately however, because of Dotty#12179, this would create an implicit encoder which would never be found. This created the need for the additional abstraction of AnyValEncoderContext and AnyValDecoderContext which would define makeMappedEncoder/makeMappedDecoder stub methods that the LowPriorityImplicits methods anyValEncoder/anyValDecoder could delegate the actual encoding/decoding work into. Hopefully when Dotty#12179 is resolved all of this convoluted logic can be removed and we can go back to the simpler implementation.