Scala 3 experiments

Small Scala 3 examples of hand-written functional typeclasses:

• Functor.scala, Applicative.scala, and Monad.scala define the abstractions and common collection helpers.
• IdentityMonad.scala and ReaderMonad.scala provide Identity and Reader instances.
• Typeclass.scala demonstrates Area and Show typeclasses with rectangle and circle instances; TypeclassDemo.scala is the runnable demonstration.
• catseffect/HelloWorld.scala is a minimal Cats Effect application.

Run the test suite with sbt test. Run an example with sbt run and select the main class when prompted (for example, typeclassDemo).