Revamped progress calculation around Task and andThen

[zwaldowski]

What's in this pull request?

Working towards #260.

Tasks created with custom progress and tasks returned from andThen with custom progress count for a unit count of 100.

Considering @piercifani's code from #260:

return startX()
    .andThen(upon: .any(), start: startY)
    .andThen(upon: .any(), start: startZ)
    .andThen(upon: .any(), start: doAA)
    .andThen(upon: .any(), start: startAB)

We have one initial Task with four calls to andThen.

At the moment the above code returns, this will now look like:

<BNRTaskRootProgress: 0x600002061220> : Parent: 0x0 / Fraction completed: 0.0000 / Completed: 0 of 5  
  <NSProgress: 0x600002061360> : Parent: 0x600002061220 / Fraction completed: 0.0000 / Completed: -1 of 0  

Then, let's say startY creates a Progress of its own. That'll look like:

<BNRTaskRootProgress: 0x600002061220> : Parent: 0x0 / Fraction completed: 0.1765 / Completed: 1 of 104  
  <NSProgress: 0x6000020645a0> : Parent: 0x600002061220 / Fraction completed: 0.1736 / Completed: 0 of 100  
    <NSProgress: 0x600002061360> : Parent: 0x6000020645a0 / Fraction completed: 0.0000 / Completed: 0 of 100  
    <NSProgress: 0x600002064640> : Parent: 0x6000020645a0 / Fraction completed: 0.1827 / Completed: 65536 of 358644  


<BNRTaskRootProgress: 0x600002061220> : Parent: 0x0 / Fraction completed: 0.6773 / Completed: 1 of 104  
  <NSProgress: 0x6000020645a0> : Parent: 0x600002061220 / Fraction completed: 0.7812 / Completed: 0 of 100  
    <NSProgress: 0x600002061360> : Parent: 0x6000020645a0 / Fraction completed: 0.0000 / Completed: 0 of 100  
    <NSProgress: 0x600002064640> : Parent: 0x6000020645a0 / Fraction completed: 0.8223 / Completed: 294912 of 358644  


<BNRTaskRootProgress: 0x600002061220> : Parent: 0x0 / Fraction completed: 0.9333 / Completed: 1 of 104  
  <NSProgress: 0x6000020645a0> : Parent: 0x600002061220 / Fraction completed: 0.9606 / Completed: 95 of 100  
    <NSProgress: 0x600002061360> : Parent: 0x6000020645a0 / Fraction completed: 0.2128 / Completed: 101816 of 478443  

Then, the last bits will trickle in:

<BNRTaskRootProgress: 0x600002061220> : Parent: 0x0 / Fraction completed: 0.9712 / Completed: 101 of 104  

Testing

Task progress reporting now has 100% test coverage.

• Integrate "something important like URLSessionTask"-in-the-middle test, like the example above
• Integrate "something important like URLSessionTask"-at-the-beginning test, like from Improves progress percent calculation #259 (ping @DrewFitz for a manual test against this branch).

API Changes

No public API changes, all under the hood bug fixes and cleanups.

[DrewFitz]

Did an initial test pass within my project and progress from andThening upon URLSessionTasks looks much more natural.

[piercifani]

Much much better! 👏 👏 👏