Add WhenAny(UniTask, UniTask) and WhenAll(UniTask, UniTask) overloads with 2-15 arguments

[SolidAlloy]

Implements issue #262

• made WhenAnyPromise and WhenAllPromise classes partial
• added constructors with 2-15 arguments to WhenAnyPromise and WhenAllPromise
• added UniTask.WhenAny(UniTask) and UniTask.WhenAll(UniTask) overloads with 2-15 arguments using those constructors
• added unit tests for WhenAny and WhenAll methods with three arguments. Let me know if I should test more overloads.

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[neuecc]

Reducing only the params (and adding a lot of overload for the sake of it) while allocation of the WhenAllPromise itself is not avoided does not look appealing. Let me think about this for a moment.

[popcron]

Reducing only the params (and adding a lot of overload for the sake of it) while allocation of the WhenAllPromise itself is not avoided does not look appealing. Let me think about this for a moment.

it doesnt look appealing but thats sadly the cost of getting to 0 allocations. imo it shouldnt matter since this unappealing looking code shouldnt be seen by the end user of the library. it can be seen in other c# libs that reach for minimum allocations.

[github-actions[bot]]

This PR is stale because it has been open 90 days with no activity. Remove stale label or comment or this will be closed in 7 days.

[nolimet]

while the idea looks cool. I don't understand why you did not sure params and an array. This way you only need one input. Would clean up the code by a lot

[github-actions[bot]]

This PR is stale because it has been open 90 days with no activity. Remove stale label or comment or this will be closed in 7 days.