WIP: Reduce prune memory usage

[MichaelEischer]

What does this PR change? What problem does it solve?

The main memory usage of prune (besides the repository index) is a huge CountedBlobSet. The blob ids stored in this set are already stored in the repository index. This PR contains a proof of concept for a data structure that no longer duplicates these ids, but rather reuses them.

This PR fundamentally depends on #4352, which stores all indexEntries in a large array. Thus an indexEntry and its contained blob id becomes identifiable by its array index. As an indexMap can only add but not remove entries, this array index is guaranteed to be stable. Thus the AssociatedData structure in the PR can allocate an array that only consists of the values that were stored in a CountedBlobSet and use the array index to look up the matching blob id in the indexMap.

Using this PR, prune requires up to 50% less memory while determining which blobs to keep.

Was the change previously discussed in an issue or on the forum?

Alternative to the minimal perfect hashes discussed in #3328

Checklist

• I have read the contribution guidelines.
• I have enabled maintainer edits.
• I have added tests for all code changes.
• [ ] I have added documentation for relevant changes (in the manual).
• There's a new file in changelog/unreleased/ that describes the changes for our users (see template).
• I have run gofmt on the code in all commits.
• All commit messages are formatted in the same style as the other commits in the repo.
• I'm done! This pull request is ready for review.