More IO perf improvements

[pakoito]

Optimize continueOn().continueOn() to only start the coroutine of the second one.

Make map and flatMap overriden open methods without runtime costs.

flatMapping a RaiseError yields itself

[codecov]

Codecov Report

Merging #968 into master will decrease coverage by 0.02%.
The diff coverage is 66.66%.

@@             Coverage Diff              @@
##             master     #968      +/-   ##
============================================
- Coverage     46.71%   46.69%   -0.03%     
  Complexity      649      649              
============================================
  Files           308      308              
  Lines          8012     8008       -4     
  Branches        821      820       -1     
============================================
- Hits           3743     3739       -4     
  Misses         3939     3939              
  Partials        330      330

Impacted Files	Coverage Δ	Complexity Δ
.../arrow-effects/src/main/kotlin/arrow/effects/IO.kt	68.96% <66.66%> (-2.01%)	9 <2> (ø)

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