Records counting

[onsails]

First of all thank you for this beautiful crate. I’ve already adopted it and looking forward to contribute.
I need to count record in a single chunk and in all chunks. Looking at internals I can see various ways to do this. What is the recommended way to count records?

[itsybitesyspider]

@prettynatty Hi and thanks!

I've added an example (https://github.com/itsybitesyspider/retriever/blob/master/examples/count_records.rs) of how to efficiently maintain a count of records that meet some criteria. Please let me know if for any reason it doesn't match your use case.

Eventually I want built-in implementations of common reductions, including counting records or other summation strategies. I'm going to leave this ticket open until that is done.