Allow usage of arrays in addition to variadic arguments in Interval and Plan functions

[WilsonGramer]

Although it is cleaner to use a variadic function when creating schedules (ex. via Plan.of(_ dates: Date...)), sometimes it is necessary to pass an array instead of a list of parameters, like when dealing with data obtained at runtime. This PR adds that ability. For example, the Plan.of(_:) function can now take an array of Dates like so:

let dates = [Date(), Date(), Date(), Date(), Date()]
Plan.of(dates).do { ... }

The existing API has not changed whatsoever, the only difference being that the function implementations are now located in the array-variant of the function, and the existing variadic-variant (whose parameters are interpreted as an array in the function body) now simply passes its parameters into the array-variant.

[codecov-io]

Codecov Report

Merging #25 into master will increase coverage by 0.17%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #25      +/-   ##
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+ Coverage   90.71%   90.88%   +0.17%     
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  Files          16       16              
  Lines         840      834       -6     
==========================================
- Hits          762      758       -4     
+ Misses         78       76       -2

[luoxiu]

Hi, @Wilsonator5000 , thank you for your contribution to the project!
Yes, sometimes we do need to use an array as a parameter! I saw your changes, it seems you haven't considered the case of an empty array, which could cause out of bounds errors.

[WilsonGramer]

Hi @jianstm,

Yep, I forgot to consider that. Should I implement it where the functions return nil if an empty array is given, throw an error or use a precondition (crash if empty) so the existing API doesn’t change?

Thanks,
Wilson

[luoxiu]

Just return a never plan.

[WilsonGramer]

OK, I added the checks and some corresponding unit tests. I originally made Plan equatable so I could test for never Plans in the unit tests, but it proved to be unreliable. Instead I created an isNever function that only works for never Plans. Although an Equatable check would be useful, I think it's out of the scope of this PR.

[luoxiu]

@Wilsonator5000 Thanks!