`Resource.eval` extra cancellation point

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:

• https://github.com/typelevel/cats-effect/blob/a48cac09ce5d236ff1e0857bc2e2722b3a6f82b1/kernel/shared/src/main/scala/cats/effect/kernel/Resource.scala#L193
• https://github.com/typelevel/cats-effect/blob/a48cac09ce5d236ff1e0857bc2e2722b3a6f82b1/kernel/shared/src/main/scala/cats/effect/kernel/Resource.scala#L527

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.

Start in kernel/shared/src/main/scala/cats/effect/kernel/Resource.scala at the referenced lines around 193 and 527, then run the minimized Resource.eval timeout case. Compare its cancellation behavior with Resource.make and run the existing law tests. Done means the extra cancellation point is resolved without breaking those law tests.