To add missing octal conversions in conversions modules

[NithinU2802]

Hi,

While exploring the conversions module, I noticed that some octal conversion are missing. I would like to contribute the following:

• binary_to_octal.rs
• decimal_to_octal.rs
• hexadecimal_to_octal.rs
• octal_to_hexadecimal.rs

These additions would help complete the set of number base conversions in the repository.

Please let me know if it’s okay, I’m happy to raise a PR for this.