Some memo refactors

[connortsui20]

Problem

There were a few inefficiencies in the memo table (mainly extraneous clones, and how remapping of objects like goals and expressions work). Also, some of the function docs were quite verbose.

Summary of changes

This PR cleans up some of the memo table modules style and documentation, as well as removes several extraneous clones.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 70.53571% with 33 lines in your changes missing coverage. Please review.

Project coverage is 82.6%. Comparing base (ed681b5) to head (f51e4f3).

Files with missing lines	Patch %	Lines
optd/src/memo/memory/materialize.rs	27.5%	21 Missing ⚠️
optd/src/memo/memory/helpers.rs	85.5%	12 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
optd/src/memo/memory/implementation.rs	90.8% <ø> (ø)
optd/src/memo/memory/helpers.rs	85.9% <85.5%> (-2.1%)	⬇️
optd/src/memo/memory/materialize.rs	58.4% <27.5%> (+1.1%)	⬆️

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[AlSchlo]

Looks all good to me, please ensure that nothing gets broken by running the fuzzing tests again.