Deprecate/remove `INothing`?

To follow suit with https://github.com/typelevel/fs2/pull/2870.

@bpholt had an excellent minimization in https://github.com/typelevel/feral/pull/52#issuecomment-974246880 demonstrating why we needed INothing instead of Nothing. Unfortunately, subsequent refactors made it no longer applicable and I was unable to find another one, tracked in https://github.com/typelevel/feral/issues/73.

I took a quick try at replacing INothing with Nothing. So far I encountered two issues:

• To get the Encoder[INothing] into implicit scope on Scala 3, we put it inside a companion object for INothing. We can't do this with Nothing.
  https://github.com/typelevel/feral/blob/81d1997372d5156ff2e74f9df357a43dcfd8d00f/lambda/shared/src/main/scala-3/feral/lambda/INothing.scala#L27-L28

• This compile test no longer compiles, which may mean trouble for inference when using Kleisli-based tracing.
  https://github.com/typelevel/feral/blob/81d1997372d5156ff2e74f9df357a43dcfd8d00f/lambda/shared/src/test/scala/feral/lambda/TracedHandlerSuite.scala#L39

  type mismatch;
   found   : cats.data.Kleisli[cats.effect.IO,natchez.Span[cats.effect.IO],Option[Nothing]]
   required: cats.data.Kleisli[[+A]cats.effect.IO[A],natchez.Span[[+A]cats.effect.IO[A]],Option[Result]]
  Note: Option[Nothing] <: Option[Result], but class Kleisli is invariant in type B.
  You may wish to define B as +B instead.
  

  Update: the broken compile test is only broken on Scala 2, Scala 3 seems okay.

Start by reading lambda/shared/src/main/scala-3/feral/lambda/INothing.scala and the compile test around line 39 of lambda/shared/src/test/scala/feral/lambda/TracedHandlerSuite.scala. Reproduce the Scala 2 and Scala 3 behavior, then review the linked fs2 change and feral issue 73. Done means the project has a decided, tested path for retaining, deprecating, or removing INothing.