Write analysis for concept exercise `pipers-pie`

[jiegillet]

From the design file:

• [essential] all functions are implemented with recursion
• [essential] all recursive function are tail call recursive
• [essential] the List module is not used
• [actionable] pipersPi should not call factorial or doubleFactorial, suggest to build terms iteratively instead

We probably want to use this elm-review rule to check the functions are tail recursive

[essessessess2007-crypto]

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