feat: kamt: Add public fn clear to reset and clear all entries in KAMT

[snissn]

This PR introduces a new clear method to the Kamt data structure, allowing users to reset the root to an empty node and clear all entries.

Changes:

• Added pub fn clear to Kamt to reset and clear all entries.
• Updated the test suite with test_clear to verify:
  • Entries exist before clearing.
  • Entries are removed after calling clear.
  • The Kamt remains functional post-clear.

This addition enhances usability by providing an efficient way to clear the Kamt.

This functionality is introduced to support the implementation of Transient Storage (EIP-1153) in builtin-actors, enabling efficient clearing of the KAMT while adhering to Rust's RAII expectations and best practices. The previous approach attempts of using take/delete followed by new was unsuitable in Rust due to its ownership model and memory safety guarantees. The new clear method resolves this limitation, providing a safe and efficient way to reset the KAMT without requiring a full reallocation.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.86%. Comparing base (2ded891) to head (04fee90).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2092      +/-   ##
==========================================
+ Coverage   75.76%   75.86%   +0.09%     
==========================================
  Files         154      154              
  Lines       15712    15743      +31     
==========================================
+ Hits        11904    11943      +39     
+ Misses       3808     3800       -8     

Files with missing lines	Coverage Δ
ipld/kamt/src/kamt.rs	89.09% <100.00%> (+4.30%)	⬆️

... and 3 files with indirect coverage changes

[Stebalien]

One nit but otherwise this seems reasonable.

Also, I'd love to have the same method on the HAMT/AMT.