feat: Implement conflict reconciliation for private directories

[matheus23]

TODO:

• [ ] Extract out the proptest generation from the public directory tests for re-usenot doing proptesting
• Decide whether we want to prioritize merging towards directories in the private case, too (Yes)
• [ ] Write proptests for private directory merging not doing proptesting
• Write function documentation

Notes on not doing proptesting:

• I learned a lot trying to state machine proptest private reconciliation.
• When the proptests failed, not once have I found a bug in the actual code, but always in the reference state machine that I was comparing to.
• To properly do this, I'd need to implement a close-to-full implementation of a file system CRDT. That's a lot of work. I didn't manage to find suitable simplifications yet. (I tried simplifying it to only files & obviously removing encryption, which works but is still a lot of work. I also tried "flattening" the file system to work on full paths only, but that throws away valuable information which makes it mismatch the actual file system.)
• I've considered just testing the private file system against equivalent operations in the public file system. That'd be really close to working, except for two problematic cases: (1) if two directories are concurrently created, the private filesystem has to tie-break, because they're encrypted using different keys, wheras the public file system can simply merge and (2) the tie-breaking on file content works differently, because in the public case it can just tie-break on the file content CID, but in the private file system there is no such CID.

[codecov]

Codecov Report

Attention: Patch coverage is 57.64192% with 97 lines in your changes are missing coverage. Please review.

Project coverage is 56.05%. Comparing base (a0bf23c) to head (13bcec9).

❗ Current head 13bcec9 differs from pull request most recent head 41b105a. Consider uploading reports for the commit 41b105a to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #431      +/-   ##
==========================================
+ Coverage   54.93%   56.05%   +1.12%     
==========================================
  Files          57       58       +1     
  Lines        4156     4410     +254     
  Branches     1036     1108      +72     
==========================================
+ Hits         2283     2472     +189     
- Misses       1197     1201       +4     
- Partials      676      737      +61     

Files	Coverage Δ
wnfs/src/private/forest/traits.rs	100.00% <ø> (ø)
wnfs/src/private/node/keys.rs	81.81% <100.00%> (+2.87%)	⬆️
wnfs/src/utils/common.rs	66.66% <100.00%> (+16.66%)	⬆️
wnfs/src/private/keys/privateref.rs	55.17% <0.00%> (ø)
wnfs/src/private/encrypted.rs	57.14% <0.00%> (-6.50%)	⬇️
wnfs/src/private/link.rs	56.66% <27.27%> (+2.23%)	⬆️
wnfs/src/private/node/header.rs	75.00% <69.44%> (-3.13%)	⬇️
wnfs/src/private/node/node.rs	70.76% <67.56%> (-0.32%)	⬇️
wnfs/src/private/file.rs	67.92% <53.06%> (-2.19%)	⬇️
wnfs/src/private/directory.rs	67.64% <54.87%> (-1.28%)	⬇️

... and 6 files with indirect coverage changes