MRG: misc Rust updates to core

[ctb]

A few useful things from the branchwater plugin perspective:

• add getters to num, scaled, and n_hashes in Record
• add misc documentation, based in part on descriptions in Feat: on-disk RevIndex based on RocksDB #2230
• add #[derive Clone] to Collection and CollectionSet
• add Collection::from_rocksdb(...)

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 86.68%. Comparing base (c621bd5) to head (b6640c2).
Report is 1 commits behind head on latest.

Files	Patch %	Lines
src/core/src/collection.rs	80.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           latest    #3297   +/-   ##
=======================================
  Coverage   86.67%   86.68%           
=======================================
  Files         137      137           
  Lines       15998    16003    +5     
  Branches     2728     2728           
=======================================
+ Hits        13867    13872    +5     
  Misses       1822     1822           
  Partials      309      309           

Flag	Coverage Δ
hypothesis-py	25.40% <ø> (ø)
python	92.37% <ø> (ø)
rust	62.92% <90.00%> (+0.06%)	⬆️

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[ctb]

for your delectation @luizirber

[luizirber]

👍 overall, but I think the load_sig change should be reverted or better contextualized =]

[ctb]

ready once tests pass! thx