libindex: add some guess at layer preallocation sizing

[hdonnay]

This is an idea I had that I threw together.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 89.47368% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 56.45%. Comparing base (417c2b0) to head (134bdde). Report is 4 commits behind head on main.

:exclamation: Current head 134bdde differs from pull request most recent head 5670344. Consider uploading reports for the commit 5670344 to get more accurate results

Files	Patch %	Lines
libindex/fetcher.go	89.47%	1 Missing and 1 partial :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1202      +/-   ##
==========================================
+ Coverage   55.33%   56.45%   +1.11%     
==========================================
  Files         265      233      -32     
  Lines       16574    15370    -1204     
==========================================
- Hits         9172     8677     -495     
+ Misses       6451     5807     -644     
+ Partials      951      886      -65     

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[hdonnay]

This uses the mean, but probably shouldn't. Something like a running median would be better, but would require an indexed skip list implementation.