Fix #423: Merge the TypeBounds of type members without needing subtyping.

[sjrd]

There was an infinite recursion between looking up a type member of a refinement and subtyping of that member against the same refinement. This came from computing the merged TypeBounds of the type member during subtyping, which used subtyping to get rid of useless bounds.

We break the cycle by not using subtyping when merging the TypeBounds of a type member anymore. Instead, we manually dive into possibly-higher-kinded bounds (TypeLambdas themselves, but also Nothing and AnyKind), and otherwise construction a union or intersection type.

Unwrapping higher-kinded bounds is necessary because constructing a uniont or intersection requires proper types.

---

I did not manage to come up with an isolated reproduction, unfortunately. But with the dependency in #423, I can now successfully run the following test:

  testWithContext("issue-423") {
    val FlowOpsClass = ctx.findTopLevelClass("akka.stream.scaladsl.FlowOps")

    val groupBy = FlowOpsClass.findAllOverloadedDecls(termName("groupBy")).find {
      groupBy => groupBy.paramSymss.map(_.merge.size).sum == 4
    }.get
    println(groupBy.tree.get.showMultiline)

    val select = findTree(groupBy.tree.get) {
      case select @ Select(_, SignedName(SimpleName("via"), _, _)) => select
    }
    println(select.showBasic)
    println(select.symbol)
  }

[adpi2]

It passes my test and does not break anything else