feat: unique for arrays

[akhercha]

Pull Request type

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Build-related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

Issue number #172

What is the new behavior?

• Added the unique methods for SpanTraitExt & ArrayTraitExt,
• Added the corresponding unit tests.

Does this introduce a breaking change?

• Yes
• No

Other information

For this little snippet:

let shortest: Span<T> = if (self.len() <= 1) || (ret.len() <= self.len()) {
    ret.span()
} else {
    self
};

Without the self.len() <= 1 member some of my tests were failing.
Tried multiple things but couldn't find any better even though I feel like there's a more optimized solution... Curious to have your inputs! 🙏

[gaetbout]

Maybe it is best to keep the rust name (unique) as dev might be more used to that one?