IORunLoop#step bug

[nomisRev]

After a chat on Gitter conversation a PR to fix the following issue.

import cats.effects.IO

IO.pure(1).uncancelable.unsafeRunSync()

// Exception in thread "main" java.lang.AssertionError: unreachable
//  at cats.effect.IO.unsafeRunTimed(IO.scala:328)
//  at cats.effect.IO.unsafeRunSync(IO.scala:240)

This is caused because the callstack is still empty at that point in IORunLoop#step and as an optimization the currentIO doesn't get wrapped in Async.
This causes unsafeRunTimed to run into case _ => throw AssertionError("unreachable")

[codecov-io]

Codecov Report

Merging #533 into master will decrease coverage by 0.18%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #533      +/-   ##
==========================================
- Coverage   89.77%   89.59%   -0.19%     
==========================================
  Files          71       71              
  Lines        2054     2056       +2     
  Branches      147      132      -15     
==========================================
- Hits         1844     1842       -2     
- Misses        210      214       +4