chore(api): mutable API preliminary work

[appcypher]

Summary

Initial work on mutable/ root tree API. There are still some work that needs to be done to complete the API (public/private root merge, blockstore ref only, etc.) but this is as far as we can go right now.

Test plan (required)

scripts/rs-wnfs.sh test

Closing issues

[appcypher]

The unused lint errors can be ignored since the API is not public yet.

[codecov]

Codecov Report

Merging #229 (5fa7ac1) into main (2f8ad3f) will increase coverage by 0.20%.
The diff coverage is 63.76%.

@@            Coverage Diff             @@
##             main     #229      +/-   ##
==========================================
+ Coverage   60.20%   60.41%   +0.20%     
==========================================
  Files          38       40       +2     
  Lines        2699     2766      +67     
  Branches      664      687      +23     
==========================================
+ Hits         1625     1671      +46     
- Misses        623      625       +2     
- Partials      451      470      +19     

Impacted Files	Coverage Δ
wnfs/examples/private.rs	0.00% <0.00%> (ø)
wnfs/src/private/forest.rs	68.18% <ø> (+4.54%)	⬆️
wnfs/src/traits.rs	0.00% <0.00%> (ø)
wnfs/src/root_tree.rs	67.69% <67.69%> (ø)