leap (concept exercise): replace with other concept exercise

[ErikSchierboom]

One of the wip concept exercises is leap, which is actually a practice exercise. As we do not allow practice exercises to be converted to concept exercise, it should be deprecated and replaced with another concept exercise.

The concepts it teaches are:

• arithmetic-operators
• integer-equality
• boolean-expressions

Candidate exercises to port

• Elixir: freelancer-rates (also includes a floating point part)
• Common Lisp: pizza-pi
• C#: cars-assemble (also includes a floating point part)
• Haskell: temperature (also includes a floating point part)

See #563 for context

[SaschaMann]

One of the wip concept exercises is leap, which is actually a practice exercise. As we do not allow practice exercises to be converted to concept exercise, it should be deprecated and replaced with another concept exercise.

There was previous discussion that this is allowed as an exception, see e.g. #365

[ErikSchierboom]

I'll discuss it with @iHiD

[SaschaMann]

For completeness I just want to mention that #365 doesn't technically convert it but rather deprecates the practice exercise and creates a new concept exercise with a new slug.