[WIP] Rewrite to Scala 3

[sergeykolbasov]

Done:

• Get rid of HList in favor of new Tuple
• Get rid of Witness.T in favor of native literal types & ValueOf
• Get rid of magnet pattern for get("hello") { } apply methods, thanks to the new fancy @targetName. That API looks much user-friendlier now, see Mappable. I had to do some repetitive work, but it's still cleaner for the users I hope.
• Drop ToAsync stuff that enabled Scala & Twitter Futures in Mappable. I don't think the HTTP library should do it anyway. If users want to use different effects in their app, they shall convert between them explicitly.
• Drop the iteratee support
• Still using Finagle & Shapeless from 2.13. We still need shapeless for coproducts, because union types are meh.

TODO:

• Fix compilation & specs outside of core
• Use the latest libraries
• Adjust core to cats-effect 3

[sergeykolbasov]

Ideally, I'd move builds & releases to Github Action, but I need support from @vkostyukov for that

[spockz]

Still using Finagle & Shapeless from 2.13. We still need shapeless for coproducts, because union types are meh.

@sergeykolbasov I haven't worked with scala 3 too much but union types got me excited. Can you share why they are "meh"?

[sergeykolbasov]

@spockz Yeah, I got excited as well. Later I found out that due to their commutative nature it's impossible to define a proper type classes derivation.

It's also untagged comparing to Coproduct, so if you have generic union:

def test[A, B](v: A | B): Unit = v match {
  case a: A => println(a)
  case b: B => println(b)
}

It's pretty much useless pattern matching without macrosing all the way down due to type erasure. I've hacked some macro indeed, but there were issues when A | A | B was smashed into A | B during macro compilation. And it is kinda unacceptable in finch logic. So Coproduct it is.

[sergeykolbasov]

Update:
Completed the migration to CE3 and refurbished the rest of the project.

The issue I'm facing now is that circe for Scala 3 doesn't support incomplete JSON decoding, so our examples fail to compile. Trying to figure it out now.

[joroKr21]

Why do we even need Coproduct? It can easily be replaced by OneOf[Tuple] and an integer.