[REPL] Add show capability to common types

[felixmulder]

This PR adds a trait Show[-T] to the Dotty library in order to print things in a better way for both the REPL and potentially other serializations. It also makes sure that the REPL shows all variables instead of toStringing them directly.

For now, there's a default show instance that calls toString so that there's a fallback in case of a Show instance not existing for a particular type. But potentially we could disallow printing things without a Show instance in the future...

Since we don't have control over the stdlib at the moment, we're stuck with providing default typeclass instances in the dotty.Show object. But ideally once @OlivierBlanvillain's HList implementation is in, perhaps we should investigate making a Show instance for DottyProduct or something similar so that we can have showing of all case classes 🎈

A known shortcoming in this PR is with regards to union types in e.g:

scala> List(1, 2, "three")
val res0: List[Int | String] = List(1, 2, three)
// instead of: List(1, 2, "three")

I tried to make the default instance down cast things if possible, but that put me into overflows for certain cases. Perhaps I could try harder if we really want this before bootstrapped library...

review: @odersky | @smarter

merging this solves: #1681

[OlivierBlanvillain]

Did you try defining Show[Nil] and Show[None] instead?

[felixmulder]

I did, but I didn't push it! (=

It actually gives a better representation of list:

scala> Nil
val res0: scala.collection.immutable.Nil.type = Nil
scala> List()
val res1: List[Nothing] = List()

instead of printing Nil for both.

[smarter]

See also the discussion in scala/scala#5222

[odersky]

LGTM 👍