Adds part06 featuring modules and doc tests

codes/rust_lang/part02/src/main.rs

codes/rust_lang/part04/Cargo.toml

codes/rust_lang/part04/src/main.rs

codes/rust_lang/part05/Cargo.toml

codes/rust_lang/part05/src/main.rs

codes/rust_lang/part06/Cargo.toml

codes/rust_lang/part06/src/io.rs

codes/rust_lang/part06/src/lib.rs

codes/rust_lang/part06/src/main.rs

codes/rust_lang/part06/src/minimum.rs

codes/rust_lang/part06/src/something_or_nothing.rs