feat: Preliminary RootTree APIs

[matheus23]

With these two PRs:

• feat: Implement wnfs-unixfs-file crate for encoding big byte arrays in IPLD #375
• Match the public and private APIs for content reading #386

it's now possible to have a single root API for both the public and private file system!

This implements it.

---

• Expose the RootTree API
• Fix compilation of wnfs-nameaccumulator when the num-bigint-dig feature is disabled
• Don't use .unwrap() in AccessKey APIs and make serializing/deserializing fallible
• Remove some println!s that got left over from debugging

[codecov]

Codecov Report

Attention: Patch coverage is 59.61538% with 42 lines in your changes are missing coverage. Please review.

Project coverage is 54.35%. Comparing base (87839b7) to head (be9def5).

❗ Current head be9def5 differs from pull request most recent head 2b12119. Consider uploading reports for the commit 2b12119 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #390      +/-   ##
==========================================
- Coverage   54.57%   54.35%   -0.22%     
==========================================
  Files          57       57              
  Lines        3956     4003      +47     
  Branches      963      980      +17     
==========================================
+ Hits         2159     2176      +17     
- Misses       1175     1191      +16     
- Partials      622      636      +14     

Files	Coverage Δ
wnfs-nameaccumulator/src/traits.rs	66.66% <ø> (-11.67%)	⬇️
wnfs/src/lib.rs	90.00% <ø> (ø)
wnfs/src/private/node/keys.rs	78.94% <ø> (ø)
wnfs/src/private/previous.rs	71.18% <ø> (ø)
wnfs/src/public/directory.rs	67.60% <ø> (ø)
wnfs/src/private/keys/access.rs	57.57% <0.00%> (+3.29%)	⬆️
wnfs/src/public/file.rs	66.39% <20.00%> (-2.58%)	⬇️
wnfs/src/root_tree.rs	63.15% <62.88%> (-0.23%)	⬇️

... and 3 files with indirect coverage changes