added header and safety note from Sha256 docs
Merge pull request #133 from pfalabella/sha-header

added header and safety note from Sha256 docs
Make 100 Doors problem's solution more functional.
Merge pull request #134 from utkarshkukreti/make-100-doors-functional

Make 100 Doors problem's solution more functional.
Use `AdditiveIterator`'s `sum()` instead of normal `fold()`.
Remove unnecessary type annotation.
`from_str` is in the prelude. No need to `use` it.
`stdin()` returns a `BufferedReader` already; no need to wrap it again.
Remove unnecessary imports.
Merge pull request #135 from utkarshkukreti/refactor-arithmetic-integers

Refactor arithmetic integers
Merge pull request #136 from utkarshkukreti/refactor-arithmetic-mean

Use `AdditiveIterator`'s `sum()` instead of normal `fold()`.
Make Binary Digits' solution more functional.
Merge pull request #137 from utkarshkukreti/refactor-binary-digits

Make Binary Digits' solution more functional.
`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
Added power set implementation