IndexedStateT is stack unsafe for left associative binds

[alexandru]

This test will fail with a stack overflow:

test("flatMap is stack safe on repeated left binds when F is") {
  val unit = StateT.pure[Eval, Unit, Unit](())
  val result = (0 until 100000).foldLeft(unit) { (acc, _) =>
    acc.flatMap(_ => unit)
  }
  result.run(()).value should === (((), ()))
}

Note that the usual 10,000 iterations that we've been using for testing stack safety aren't enough, because for IndexedStateT the stack size doesn't grow that fast. But the error can be seen for 100,000.

This is a problem for projects like cats-effect that need to provide a lawful Sync[StateT[F, S, ?]] instance.

NOTE: similar with #1733

[alexandru]

And the PR is now here: #2187