Prevent Future.parallel from cancelling settled computations

[Avaq]

This was a terribly difficult bug to track down. You may be interested to see this @stevenyap:

• Future.parallel sends a cancel signal to futures that reject
• Some _fork methods cannot deal with that. In particular: Future.hook does not expect to be cancelled right after being rejected, and this lead to an unwanted resolution signal being sent.
• The resolution signal would cause problems in the sequence interpreter. In particular: The interpreter would already be "done" and cleaned up its state when receiving the unexpected resolution signal. This in turn would cause it to try and re-resolve the current action, which finally left us with action.resolve throwing an error.

Fixes #123

[codecov-io]

Codecov Report

Merging #124 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #124   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          47     47           
  Lines        1086   1088    +2     
=====================================
+ Hits         1086   1088    +2

Impacted Files	Coverage Δ
src/parallel.js	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3905da6...720b298. Read the comment docs.

[davidchambers]

Nice pull request description, Aldwin! Making such notes contemporaneously is a good idea. :)