Rot13 rust

[michelsmartinez]

Description

This program (lib) make the pseudo encryption 'rot13' on strings, using Rust.
ROT13 ("rotate by 13 places", sometimes hyphenated ROT-13) is a simple letter substitution cipher that replaces a letter with the 13th letter after it, in the alphabet.

Checklist:

• My code follows the style guidelines of this project
• I have added my code in its respective language folder.
• I have mentioned a short description about the code at the beginning of the program.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• My changes generate no new warnings.
• I have not deleted any files that already exist in the repository.

This is for the issue #3