TopDownIndex doesn't retain order of operations

[kubukoz]

Related: #591 / #602 - the getOperations() method has been made to retain the order of operation in the service's original definition, but TopDownindex.getContainedOperations() didn't get the same treatment.

This results in unpredictable ordering, which has the same flaws as the previous behavior of getOperations.

For example, you can see the ordering change in the output of the following Scala script:

//> using scala 3.7.1
//> using dep software.amazon.smithy:smithy-model:1.60.3
//> using option -Wunused:all
import software.amazon.smithy.model.Model
import software.amazon.smithy.model.shapes.ShapeId
import software.amazon.smithy.model.knowledge.TopDownIndex
import software.amazon.smithy.model.shapes.ServiceShape

val m = Model
  .assembler()
  .addUnparsedModel(
    "test.smithy",
    """$version: "2"
      |namespace demo
      |
      |service DummyService {
      |    version: "0.0"
      |    operations: [
      |        DummyHostPrefix
      |        Dummy
      |        DummyPath
      |    ]
      |}
      |
      |
      |operation DummyHostPrefix {}
      |operation Dummy {}
      |operation DummyPath {}
      |""".stripMargin,
  )
  .assemble()
  .unwrap()
// m: Model = software.amazon.smithy.model.Model@90925b8

TopDownIndex
  .of(m)
  .getContainedOperations(ShapeId.from("demo#DummyService"))
// res0: Set[OperationShape] = [(operation: `demo#Dummy`), (operation: `demo#DummyHostPrefix`), (operation: `demo#DummyPath`)]

m.expectShape(ShapeId.from("demo#DummyService"), classOf[ServiceShape]).getOperations()
// res1: Set[ShapeId] = [demo#DummyHostPrefix, demo#Dummy, demo#DummyPath]

[mtdowling]

Looks like we're using TreeSet instead of LinkedHashset. It's ...probably safe to change it? We'll try and see if anything blows up :)