fix: More reliably cache NameAccumulator modexps

[matheus23]

BigUint::modexp operations take up to 14ms in Wasm on my machine, and it's the core operation for NameAccumulators.

This introduces a LRU cache inside HamtForest that caches NameAccumulator operations.

This is much more reliable than sprinkling OnceCells every here and there, since

1. The "eviction" of these OnceCell caches depends on the lifetime of the object they're contained in, and it's harder to make these structs longer-lived in some cases.
2. Sometimes the structs that these OnceCells are contained inside are mutable via &mut self operations. We need to remember to always empty these OnceCells when that happens. This is bad for two reasons: (1) we might forget doing so sometimes, keeping stale, incorrectly cached data around and (2) even though the cache doesn't match the object anymore, it may still be a useful cache!

[codecov]

Codecov Report

Merging #326 (e4d26f9) into main (aa44005) will decrease coverage by 0.04%.
The diff coverage is 68.65%.

@@            Coverage Diff             @@
##             main     #326      +/-   ##
==========================================
- Coverage   56.30%   56.27%   -0.04%     
==========================================
  Files          43       43              
  Lines        3204     3172      -32     
  Branches      773      770       -3     
==========================================
- Hits         1804     1785      -19     
+ Misses        904      892      -12     
+ Partials      496      495       -1     

Files Changed	Coverage Δ
wnfs/examples/write_proofs.rs	0.00% <0.00%> (ø)
wnfs/src/private/link.rs	51.89% <ø> (+2.50%)	⬆️
wnfs/src/private/forest/proofs.rs	53.22% <57.14%> (-1.78%)	⬇️
wnfs/src/private/forest/hamt.rs	44.76% <61.90%> (+1.60%)	⬆️
wnfs-nameaccumulator/src/name.rs	81.06% <66.66%> (-1.45%)	⬇️
wnfs/src/private/node/header.rs	79.10% <66.66%> (-1.72%)	⬇️
wnfs/src/private/directory.rs	68.44% <75.00%> (+0.65%)	⬆️
wnfs/src/private/file.rs	80.06% <83.33%> (+1.15%)	⬆️
wnfs/src/private/node/node.rs	71.25% <83.33%> (+1.31%)	⬆️
wnfs/src/private/forest/traits.rs	100.00% <100.00%> (ø)
... and 2 more

[appcypher]

This looks great to go. Another performance boost.
Great stuff @matheus23. We slowly getting there.