Reduce constant pool usage, this make Magnolia able to derive instance for sealed trait that has a lot of subtypes

[nox213]

Even after #485 has been merged, I found that there is a limit to the number of subtypes when deriving instances with the split. By splitting subtypes array initialization to multiple functions, we are able to avoid method too large error, but we aren't able to avoid class too large error because of constant pool limit. When we initialize Subtype, we pass a lot of values, so we consume constant pool a lot which has 65534 entries. When I tested like below, the limitation of the number of subtypes is about 1550 with current implementation.

trait Print {
 ...
}

trait GenericPrint {
   ... 
}

sealed trait Event

case class AEvent(...) extends Event
... // variants

object Instances extends GenericPrint {
    implicit val aEventInstance = ...
    ... // instances of subtypes

    implicit val eventInstance = gen[Event]
}

With this PR, we can derive instance for type has about 3600 subtypes and it seems there is no compilation speed degradation.

[adamw]

Thanks :)

[adamw]

@nox213 Sorry but I have to revert this one, it causes some serialization problems (see https://github.com/softwaremill/magnolia/issues/487). Maybe it would be possible for you or @RustedBones to fix them, then we can merge this (patched) change again :)

[nox213]

I see. Thank you for letting me know. I will look into it.