fix: Improve performance of get_revision_name()

[matheus23]

This simply caches the result of get_revision_name(), as that's accessed quite a lot and would otherwise cause lots of repeated BigUint operations, which are costly.

It improves performance of general filesystem operations by roughly 20% in my limited testing (I compared runtime of cargo test --lib -p wnfs -- "private::directory::").

[codecov]

Codecov Report

Merging #317 (170d0ee) into main (e998628) will increase coverage by 0.09%.
The diff coverage is 95.23%.

@@            Coverage Diff             @@
##             main     #317      +/-   ##
==========================================
+ Coverage   56.25%   56.34%   +0.09%     
==========================================
  Files          43       43              
  Lines        3198     3207       +9     
  Branches      774      775       +1     
==========================================
+ Hits         1799     1807       +8     
  Misses        903      903              
- Partials      496      497       +1     

Files Changed	Coverage Δ
wnfs/src/private/node/header.rs	80.82% <93.33%> (+1.13%)	⬆️
wnfs/src/private/directory.rs	67.78% <100.00%> (ø)
wnfs/src/private/file.rs	78.37% <100.00%> (ø)
wnfs/src/private/node/node.rs	70.11% <100.00%> (ø)
wnfs/src/private/previous.rs	70.78% <100.00%> (ø)

[appcypher]

LGMT! 🎉