`from_str` is in the prelude; no need to explicitly `use` it.
Simplify `perfect_number` function.
Merge pull request #139 from utkarshkukreti/refactor-perfect-numbers

Simplify `perfect_number` function.
Merge pull request #138 from utkarshkukreti/dont-import-from_str

`from_str` is in the prelude; no need to explicitly `use` it.
String concatenation
Merge pull request #140 from JakeMick/string_concatenation

String concatenation