/// # strlen
/// Returns str.len().
///
/// This function has no reason to exist.
///
/// ## Example
/// ```
/// assert_eq!(ft::strlen("abc"), 3);
/// assert_eq!(ft::strlen("\0bc"), 3);
/// ```
/// # strlen_naive
/// Returns amount of sumbols to first null-symbol in the string.
///
/// ## Example
/// ```
/// assert_eq!(ft::strlen_naive("abc"), 3);
/// assert_eq!(ft::strlen_naive("a\0c"), 1);
/// ```