There are a couple of reasons this might be a good idea.

Currently it's possible to observe a difference when calling
`.map(identity)` on a `Free` instance:

```scala
scala> import cats.free._; import cats.implicits._
import cats.free._
import cats.implicits._

scala> def f(x: Free[Option, Int]) = x match {
     |   case Free.Pure(i) => i
     |   case _ => 0
     | }
f: (x: cats.free.Free[Option,Int])Int

scala> val x: Free[Option, Int] = Free.pure(3)
x: cats.free.Free[Option,Int] = Pure(3)

scala> f(x)
res0: Int = 3

scala> f(x.map(identity))
res1: Int = 0
```

Making these private also frees us up to change the internal
representation in the future without breaking user code.

Also make TrampolineFunctions package-private, since it's just a
workaround for a scalac bug and isn't really intended to be used
directly.