feat: new in slice matcher

[MakDon]

This PR provides a new matcher that returns true if the given value Eq one of the want elements.

// Example usage:
m := In([]int{1,2})
m.Matches(1) // returns true
m.Matches(2) // returns true
m.Matches(3) // returns false

[codyoss]

@MakDon Can you please open up an issue for discussion about adding such a feature. Thanks.

[codyoss]

See comment above.

[MakDon]

@codyoss Hi codyoss, thanks for your review.

I've opened up an issue #590 for further discussion.

[codyoss]

Closing this for now until discussion on the issue has settled. I will reopen if we decide to move forward with these changes.