[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[bot]]

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 :warning:
core/store/src/trie/mem/mem_tries.rs	81.25%	4 Missing and 5 partials :warning:
core/store/src/trie/mod.rs	50.00%	0 Missing and 2 partials :warning:
core/store/src/trie/mem/updating.rs	97.29%	1 Missing :warning:

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/near/nearcore/pull/12073/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=near) | Coverage Δ | | |---|---|---| | [backward-compatibility](https://app.codecov.io/gh/near/nearcore/pull/12073/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=near) | `0.17% <0.00%> (+<0.01%)` | :arrow_up: | | [db-migration](https://app.codecov.io/gh/near/nearcore/pull/12073/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=near) | `0.17% <0.00%> (+<0.01%)` | :arrow_up: | | [genesis-check](https://app.codecov.io/gh/near/nearcore/pull/12073/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=near) | `1.26% <0.00%> (+<0.01%)` | :arrow_up: | | [integration-tests](https://app.codecov.io/gh/near/nearcore/pull/12073/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=near) | `38.70% <67.15%> (-0.03%)` | :arrow_down: | | [linux](https://app.codecov.io/gh/near/nearcore/pull/12073/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=near) | `71.20% <80.29%> (+<0.01%)` | :arrow_up: | | [linux-nightly](https://app.codecov.io/gh/near/nearcore/pull/12073/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=near) | `71.12% <80.29%> (-0.02%)` | :arrow_down: | | [macos](https://app.codecov.io/gh/near/nearcore/pull/12073/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=near) | `52.82% <79.56%> (-1.24%)` | :arrow_down: | | [pytests](https://app.codecov.io/gh/near/nearcore/pull/12073/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=near) | `1.53% <0.00%> (+<0.01%)` | :arrow_up: | | [sanity-checks](https://app.codecov.io/gh/near/nearcore/pull/12073/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=near) | `1.33% <0.00%> (+<0.01%)` | :arrow_up: | | [unittests](https://app.codecov.io/gh/near/nearcore/pull/12073/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=near) | `65.37% <79.56%> (-0.03%)` | :arrow_down: | | [upgradability](https://app.codecov.io/gh/near/nearcore/pull/12073/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=near) | `0.22% <0.00%> (+<0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=near#carryforward-flags-in-the-pull-request-comment) to find out more.

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