[Sept Sprint] New exercise teaching functions

[ErikSchierboom]

This is part of our September Spring to get the Go Track in shape. Please read the Overview issue for more details. This exercise is unclaimed. To claim it, please comment below.

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Before building this exercise (or exercises), please read up on the documentation.

We need an exercise for the functions concept, which should teach about:

• Arguments (incl. syntax for arguments of the same type)
• Return values
• Named return values
• Naked return
• pass by value but the value can be a pointer
• Can also include: Anonymous functions/callbacks

It should not teach (these will be covered later):

• Variadic functions (that should be introduced with rest-and-spread)

It is worth to look if it is possible to port this exercise from the JS lasagna master exercise.

[bobtfish]

I'll take this. I plan to try using the JS lasagna master exercise and I plan to not cover anonymous functions - I think that those deserve their own concept / introduction - trying to cram them in here is biting off too much at once.

[ErikSchierboom]

@bobtfish Let me know if there is anything I can do to help.

[bobtfish]

@bobtfish Let me know if there is anything I can do to help.

PR ready for initial review in #1720 - I think it's a reasonable first take, but any/all feedback appreciated as this is my first fill concept/exercise!