Fixed check_file.rs

The previous implementation did not print anything, because it used a map without iterating through it.
Added link to rosettacode.org in rot13.rs
Added link to rosettacode.org in mutual_recursion.rs
Merge pull request #40 from aochagavia/pr-4

Fixed check_file.rs and added some links to rosettacode.org
Simplified swap.rs
Merge pull request #39 from aochagavia/pr-3

Simplified swap.rs
Use unwrap instead of ok