Remove dependency on go-cmp

[asmaloney]

Because this is a library, users have to carry around all dependencies, so the fewer the better.

I did a cursory look - is go-cmp only used in tests?

Can you get away with regular comparisons and/or using reflect.DeepEqual or is there a real need for something different?

[erni27]

No real need to use it. I've been using imcache internally in projects already having go-cmp dependency so I've missed it. Thanks!

[asmaloney]

👍 "No dependencies" is a good feature!

(If I can sneak in another thing 😄 - it might be useful to compare imcache vs. other similar libraries in the README? I'm thinking features/benefits/drawbacks, not benchmarks.)

Thank you for taking the time to put this together!

[erni27]

Yes, I agree. It would be useful. I believe users looking for an in-memory cache library can be unaware of the differences between them just by looking at their APIs so would be great to be explicit about it.

EDIT: I've created a new issue for that. #20