More aggregate functions

[Pekka Enberg]

We have machinery for aggregate functions, but only sum() and avg() are implemented now. Let's add support for rest of the aggregate functions:

https://github.com/penberg/limbo/blob/main/docs/sqlite-compat.md#aggregate-functions

[Vivek Khatri]

I have raised a PR: #100 for the aggregate function: count

Please review once. I am also attaching a screenshot of the count fn:

[mason]

Hi @penberg! I've raised a PR to try and implement the max aggregate function. I'm new to rust and contributing to database systems in general, do let me know if there is anything I can improve on!

Thank you!

[Pekka Enberg]

We now support all aggregate functions. Closing the issue, thanks everyone!