WIP: add manifest summarization

[bluegenes]

I went down a rabbithole while trying to write tests for safer selection... ended up implementing manifest summarization in rust. The output of write_summary mimics our current python manifest summarization (https://github.com/sourmash-bio/sourmash/blob/latest/src/sourmash/sig/__main__.py#L1309-L1407).

Since disk_revindex (rocksdb) are also collections with manifests, could we use this to summarize those?

[codecov]

Codecov Report

Attention: Patch coverage is 94.87179% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 86.91%. Comparing base (e0ed4c3) to head (3b17a9e).

Files	Patch %	Lines
src/core/src/collection.rs	50.00%	1 Missing ⚠️
src/core/src/manifest.rs	97.29%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           latest    #3030      +/-   ##
==========================================
+ Coverage   86.89%   86.91%   +0.01%     
==========================================
  Files         136      136              
  Lines       15542    15581      +39     
  Branches     2637     2637              
==========================================
+ Hits        13505    13542      +37     
- Misses       1735     1737       +2     
  Partials      302      302              

Flag	Coverage Δ
hypothesis-py	25.44% <ø> (ø)
python	92.87% <ø> (ø)
rust	61.90% <94.87%> (+0.43%)	⬆️

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