Address all remaining `Wx-partial` warnings

[slotThe]

Fixes (hopefully): https://github.com/xmonad/xmonad-contrib/issues/830 It also makes way for #838 (with which we can even check if we got everything!)

(N.b.: so many good uses for non-empty lists that can't be changed due to backwards compatibility :/)

Commit Summary

1c299d0c Fix partial uses of head

Fixes: https://github.com/xmonad/xmonad-contrib/issues/830 Related: https://github.com/xmonad/xmonad-contrib/pull/836

42179b86 X.U.EZConfig: Make readKeySequence return non-empty list

d668e4cb X.Prelude: Add takeS

Checklist

• [x] I've read CONTRIBUTING.md

• [ ] I've considered how to best test these changes (property, unit, manually, ...) and concluded: XXX

• [x] I updated the CHANGES.md file

  Where appropriate