[memtrie] Simplify MemTries interface

[shreyan-gupta]

Review this PR after #12069

Couple of changes to the memtrie interface

• New function get_iter to avoid the two step process of first getting root and then iterator for memtrie
• Simplification of the load_trie_from_flat_state function
• Remove infamous construct_root function as that was one too many layers of indirection
• New function apply_memtrie_changes
• New function lookup to simplify calls to memtrie key lookups

[codecov]

Codecov Report

Attention: Patch coverage is 80.29197% with 27 lines in your changes missing coverage. Please review.

Project coverage is 71.58%. Comparing base (167172b) to head (e6afe27).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
core/store/src/trie/mem/loading.rs	65.11%	7 Missing and 8 partials ⚠️
core/store/src/trie/mem/mem_tries.rs	81.25%	4 Missing and 5 partials ⚠️
core/store/src/trie/mod.rs	50.00%	0 Missing and 2 partials ⚠️
core/store/src/trie/mem/updating.rs	97.29%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #12073      +/-   ##
==========================================
- Coverage   71.60%   71.58%   -0.03%     
==========================================
  Files         816      816              
  Lines      164374   164337      -37     
  Branches   164374   164337      -37     
==========================================
- Hits       117705   117645      -60     
- Misses      41526    41555      +29     
+ Partials     5143     5137       -6     

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	38.70% <67.15%> (-0.03%)	⬇️
linux	71.20% <80.29%> (+<0.01%)	⬆️
linux-nightly	71.12% <80.29%> (-0.02%)	⬇️
macos	52.82% <79.56%> (-1.24%)	⬇️
pytests	1.53% <0.00%> (+<0.01%)	⬆️
sanity-checks	1.33% <0.00%> (+<0.01%)	⬆️
unittests	65.37% <79.56%> (-0.03%)	⬇️
upgradability	0.22% <0.00%> (+<0.01%)	⬆️

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