Add a solution for "94. An arithmetic puzzle." in 99 Problems

[dracconi]

Changes Made

Added a solution to the 94th problem in 99 Problems.
Seems to work, but who knows :D

Done per request for commits "Please contribute more solutions or improve the existing ones."

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[patricoferris]

Hi @dracconi thanks for the PR, I haven't checked your solution (I'm not brilliant at algorithms) but could you maybe refactor the code to follow the pattern many of the others follow which uses the > to indent the solution and then reuses the implemented function on some of the examples in a code block below ? This will render quite nicely on the front-end then https://ocaml.org/learn/tutorials/99problems.html and also will render the output of a "top level" function that takes as input the arguments described in the text such as:

```ocamltop
f [2; 3; 5; 7; 11];;
```

This will then output on the site (if the solution is correct ;)) as something like:

f [2; 3; 5; 7; 11];;
- : string = "2 - 3 + 5 + 7 = 11"

Thanks :))

[dracconi]

Hi @patricoferris, I've commited the updated thing per request, though I'm not sure if the example should be exactly the same as listed in the example [2; 3; 5; 7; 11], or can it be different?