Add Fetches.NumRecords(), Fetches.Empty()

[un000]

#188 (comment)

Following pattern allocates a new slice under the hood.

if len(fetches.Records()) == 0 {
		return nil
}

Add helper methods which calculates total fetch size without allocation.

@twmb

[twmb]

Update on where I'm at, I'm going to try to solve #190 before preparing for the next release and merging this, but that issue has proven to be quite complicated.

[twmb]

Thanks, I'm going to merge this. I might modify a doc line a bit locally, but this is good.

[twmb]

This has been released in v1.7.0