Initial version of the first 3 exercises of chapter 6.
Use wildcards for unnecessary variables.
Add Exercise 4.
Fix `pow` to handle power of 0.
Fix `pow` function.
Change unused variable to wildcard.
Add solution for Exercise 5.
Add solutions for Exercise 6.
Fix `msort` to handle the empty list.
Use newly defined `take'` function.
Merge branch 'ch06'