Add zebra-puzzle exercise

[glennj]

I'm sure there's an "eleganter" way solve this, but ...

[glennj]

Looking at the other languages this would normally be a function for each of the water drinker and the zebra owner. How come you have opted for a data structure holding both?

Once the puzzle is solved (once), both answers are known. Having to solve it twice is redundant.

I chose one test function with 3 assertions.

[lpil]

Please switch over to having two functions, as that's what the problem specification repo instructs. Thank you

[glennj]

Reworked as requested.

custom-types is no longer a prerequisite, as that's an implementation detail.