Make private forest multivalue

[matheus23]

This PR only changes the data format, but minimally changes the actual data stored. Specifically: We never actually generate multivalues in rs-wnfs. So why add support for them?
The reason is being future-proof: We know we'll make use of multivalues in the future, so we add support for them today, so that today's version of rs-wnfs will have some support for them even when future versions of rs-wnfs actually write multivalues.

In this PR:

• the PrivateForest type alias changed to use BTreeSet<Cid> as value, instead of just Cid
• PrivateForest.set was renamed to put, and it adds to the multivalues at a given key, instead of replacing. There's a performance optimization opportunity here to avoid walking the HAMT twice by creating a modify/upsert implementation in hamt/node.rs. But that turned out to be quite some effort, so I'm postponing that for now.
• PrivateForest.get now expects a resolve_bias function parameter. Today that mostly ends up being BTreeSet::first, in order to fetch the smallest CID. That's the tie-breaking that's specified for WNFS by default, but that will be something different e.g. when we have previous pointers, then we'll bias towards those pointers.
• PrivateForest.get_encrypted returns the whole BTreeSet now.
• There's some helper functions in PrivateForest like resolve_lowest, resolve_single and resolve_one_of that are supposed to be helpers for use with PrivateForest.get.

Closes issue: #74

[codecov]

Codecov Report

Merging #78 (d1276f1) into main (11b73a7) will increase coverage by 1.58%.
The diff coverage is 35.41%.

@@            Coverage Diff             @@
##             main      #78      +/-   ##
==========================================
+ Coverage   64.56%   66.15%   +1.58%     
==========================================
  Files          25       26       +1     
  Lines        1795     1873      +78     
  Branches      458      480      +22     
==========================================
+ Hits         1159     1239      +80     
- Misses        235      240       +5     
+ Partials      401      394       -7     

Impacted Files	Coverage Δ
wnfs/common/utils.rs	70.00% <ø> (ø)
wnfs/examples/private.rs	0.00% <0.00%> (ø)
wnfs/private/hamt/hamt.rs	48.57% <0.00%> (ø)
wnfs/private/key.rs	62.50% <0.00%> (+4.16%)	⬆️
wnfs/private/namefilter/bloomfilter.rs	88.57% <0.00%> (+2.85%)	⬆️
wnfs/private/previous.rs	71.26% <0.00%> (+1.14%)	⬆️
wnfs/public/directory.rs	69.96% <0.00%> (+0.70%)	⬆️
wnfs/private/directory.rs	72.82% <33.33%> (+0.16%)	⬆️
wnfs/private/forest.rs	56.33% <45.83%> (-1.85%)	⬇️
wnfs/private/node.rs	69.36% <60.00%> (+2.11%)	⬆️
... and 11 more

[appcypher]

Looks good to me! 🎉
Only minor fixes to be addressed.