make each solution a module

Seems these problems tend to refer to each other
additionally some hlint warnings are fixed
Problem11 - done
Problem12 - done
Problem13 - done
Problem14 - done
Problem15 - done
Problem16 - done
Problem17 - done
Problem18 - done
Problem19 - done
Problem20 - done
Problem21 - done
Problem22 - done
Problem23 - done
Problem24 - done
Problem25 - done
Problem26 - done
Problem27 - done
Problem28 - done
Problem31 - done