ex 3.14 append in terms of foldRight (and foldLeft using reverse)
ex 3.15 concat - first version
ex 3.15 concat - reuse of append
ex 3.16 - add1
formatting
ex. 3.17 - doubleToString
formatting on tests
ex. 3.18 - map
ex. 3.18 - map - stacksafe version
ex. 3.18 - map - stacksafe version => add compiler check
ex. 3.19 - filter
fix for Traverse.mapAccum
Merge branch 'mapAccum-fix' of https://github.com/abo64/fpinscala into fmaster