Implement previous iterator for subpaths

[matheus23]

WIP because:

• Documentation
• Clearer code. (Maybe some ideas for cleaning up? Perhaps let-else statements?)
• Add reference to PrivateForest in PrivateNodePathHistory?
• WASM bindings?
• git conflicts

[codecov]

Codecov Report

Merging #70 (36016b0) into main (c88a0f8) will increase coverage by 0.33%.
The diff coverage is 66.84%.

@@            Coverage Diff             @@
##             main      #70      +/-   ##
==========================================
+ Coverage   64.29%   64.62%   +0.33%     
==========================================
  Files          24       25       +1     
  Lines        1613     1795     +182     
  Branches      416      458      +42     
==========================================
+ Hits         1037     1160     +123     
- Misses        213      233      +20     
- Partials      363      402      +39     

Impacted Files	Coverage Δ
crates/fs/common/error.rs	0.00% <ø> (ø)
crates/fs/private/node.rs	67.25% <0.00%> (-3.31%)	⬇️
crates/fs/private/key.rs	58.33% <50.00%> (ø)
crates/fs/private/previous.rs	70.11% <70.11%> (ø)
crates/fs/private/hamt/node.rs	64.41% <0.00%> (+0.61%)	⬆️

[matheus23]

Flipping this to ready-for-review @appcypher, because I'm thinking this is already really useful & the wasm piece is probably another PR in and of itself, and I should probably have webnative & rs-wnfs/wasm side-by-side while working on that (i.e. it'll take a little more time).

Do you think that makes sense?

[appcypher]

Yep. Makes sense to me.

[appcypher]

Thanks for working on this 🎉. We are step closer to feature parity.
I have a few refactor comments.

[zeeshanlakhani]

Starter set of comments.

[zeeshanlakhani]

need a rebase from main, but 👍🏽. I updated an outdated comment, but no biggie there either (sorry for not realizing the node history was already in the of constructor). Thanks for all the updates.

[appcypher]

Let's go! 🎉