[memtrie] Introduce HybridArena and FrozenArena

[shreyan-gupta]

Review this PR after #12073

This PR introduces HybridArena which represents a combination of owned and shared memory.

This is useful during resharding to create temporary memtries for the children of a split parent shard.

[codecov]

Codecov Report

Attention: Patch coverage is 95.13889% with 7 lines in your changes missing coverage. Please review.

Project coverage is 71.62%. Comparing base (cf47d81) to head (68e457d).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
core/store/src/trie/mem/arena/hybrid.rs	96.52%	3 Missing and 1 partial ⚠️
core/store/src/trie/mem/arena/frozen.rs	50.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #12075      +/-   ##
==========================================
+ Coverage   71.60%   71.62%   +0.01%     
==========================================
  Files         816      818       +2     
  Lines      164337   164468     +131     
  Branches   164337   164468     +131     
==========================================
+ Hits       117676   117800     +124     
- Misses      41525    41528       +3     
- Partials     5136     5140       +4     

Flag	Coverage Δ
backward-compatibility	0.17% <0.00%> (-0.01%)	⬇️
db-migration	0.17% <0.00%> (-0.01%)	⬇️
genesis-check	1.26% <0.00%> (-0.01%)	⬇️
integration-tests	38.73% <34.02%> (+<0.01%)	⬆️
linux	71.21% <95.13%> (+<0.01%)	⬆️
linux-nightly	71.14% <95.13%> (+0.02%)	⬆️
macos	54.00% <95.13%> (-0.10%)	⬇️
pytests	1.53% <0.00%> (-0.01%)	⬇️
sanity-checks	1.33% <0.00%> (-0.01%)	⬇️
unittests	65.40% <95.13%> (+0.02%)	⬆️
upgradability	0.22% <0.00%> (-0.01%)	⬇️

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[robin-near]

Nice, this is pretty clean!