Adding docstring.

Adding instructions into docstring for resolving exercices.
Moving main functions into tests functions.

Need to be build with

rustc --test my_file.rs

Tested with:

./my_file
Problem 80 (partially)

Some parts are ugly but at least it works.
Merge branch 'docstrings' of https://github.com/darnuria/99-Problems-Rust into darnuria-docstrings

Conflicts:
	01-10/p10.rs
Correct typos in docstrings