Add memo table interface and in-memory implementation

[connortsui20]

Summary of changes

This PR puts the memo table from #104 into the main branch (copied and pasted).

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 87.92536% with 110 lines in your changes missing coverage. Please review.

Project coverage is 88.5%. Comparing base (29796e9) to head (ee81405).

Files with missing lines	Patch %	Lines
optd/src/memo/memory/implementation.rs	90.9%	41 Missing ⚠️
optd/src/memo/memory/materialize.rs	57.3%	32 Missing ⚠️
optd/src/memo/memory/helpers.rs	88.0%	26 Missing ⚠️
optd/src/memo/memory/representative.rs	50.0%	9 Missing ⚠️
optd/src/memo/memory/union_find.rs	98.6%	2 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
optd/src/memo/memory/union_find.rs	98.6% <98.6%> (ø)
optd/src/memo/memory/representative.rs	50.0% <50.0%> (ø)
optd/src/memo/memory/helpers.rs	88.0% <88.0%> (ø)
optd/src/memo/memory/materialize.rs	57.3% <57.3%> (ø)
optd/src/memo/memory/implementation.rs	90.9% <90.9%> (ø)

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[connortsui20]

Close in favor of #101