Add cooldowns

[hypergonial]

Summary

Add an easy way to handle cooldowns for custom command handler implementations, or any other usecase requiring them.

Why is this needed?

It could be handy to enforce ratelimits, or just to create custom logic within Discord bots.

Ideal implementation

Cooldown buckets that can operate on a variety of types, (perharps passing a lambda to tell it how to extract the hash so it works on any type?) and logic that can either wait for the cooldown to expire then continue or raise errors (or other similar behaviour)

Checklist

• I have searched the issue tracker and have made sure it's not a duplicate. If it is a follow up of another issue, I have specified it.

[zunda-arrow]

The lambdas are what i did for crescent. It works very well.

The rate limit should be a sliding window rate limit imo. It is the best kind honestly.