Rename RatchetKey to RevisionKey, encrypt RevisionKey within PrivateRef

[matheus23]

Closes #77

This also renames RatchetKey to RevisionKey, to be more in line with the terminology from the spec.

The RevisionKey within PrivateRefs in directories is meant to be encrypted with the directory's RevisionKey.

[matheus23]

Messed up the branching & merging. Please stand by while I fix this PR

[codecov]

Codecov Report

Merging #83 (facd248) into main (ce7d988) will decrease coverage by 0.09%.
The diff coverage is 65.51%.

@@            Coverage Diff             @@
##             main      #83      +/-   ##
==========================================
- Coverage   66.15%   66.05%   -0.10%     
==========================================
  Files          26       26              
  Lines        1873     1903      +30     
  Branches      480      486       +6     
==========================================
+ Hits         1239     1257      +18     
- Misses        240      248       +8     
- Partials      394      398       +4     

Impacted Files	Coverage Δ
wnfs/src/private/hamt/node.rs	65.64% <ø> (-0.62%)	⬇️
wnfs/src/public/directory.rs	69.96% <50.00%> (ø)
wnfs/src/public/file.rs	73.52% <50.00%> (ø)
wnfs/src/private/node.rs	68.91% <65.71%> (-0.46%)	⬇️
wnfs/src/private/directory.rs	72.50% <66.66%> (-0.33%)	⬇️
wnfs/src/private/file.rs	68.57% <66.66%> (ø)
wnfs/src/private/forest.rs	56.33% <100.00%> (ø)

[matheus23]

Flipped this to 'ready for review'. No more merge conflicts & no more rogue changes.

[zeeshanlakhani]

super minor naming thing that matters very little, 👍🏽

[appcypher]

LGTM! 🎉