`Resource.eval` extra cancellation point

[durban]

As noticed during #4625, Resource.eval seems to have an extra cancellation point at the end. I think it shouldn't. Minimization:

    "Resource.eval" in real {
      val test = Resource
        .eval(IO.uncancelable(_ => IO.sleep(100.millis)))
        .timeout(10.millis)
        .use_
      test.as(ok)
    }

This fails with the 10.millis timeout. (Note: it passes with Resource.make.) Resource.eval is documented to "preserve interruptibility of fa". Which is very much uncancellable here. Yet, it is very much cancelled. (Or rather, it is cancelled right after the uncancelable ends, I think.) The crucial points in the code seem to be:

• cats-effect/kernel/shared/src/main/scala/cats/effect/kernel/Resource.scala

  Line 193 in a48cac0

  fa.flatMap(a => continue(Resource.pure(a), stack))

• cats-effect/kernel/shared/src/main/scala/cats/effect/kernel/Resource.scala

  Line 527 in a48cac0

  fa.flatMap(a => continue(Resource.pure(a), stack, release))

The obvious fix, i.e., F.uncancelable { poll => poll(fa).flatMap(a => ...) makes a lot of existing law tests fail, so more investigation is needed.