calculate all stats for rust `linear` gather

[bluegenes]

In https://github.com/sourmash-bio/sourmash/pull/2943, I calculate all gather statistics in a function, calculate_gather_stats and use it in revindex::disk_revindex.rs.

linear gather works slightly differently, so we need a modified version of calculate_gather_stats.

Context from @luizirber:

• https://github.com/sourmash-bio/sourmash/issues/1226#issuecomment-720815108
• https://github.com/sourmash-bio/sourmash/issues/1957#issuecomment-1100711337

<- this one was relevant for in-memory revindex, but not so useful for on-disk revindex?