Refactor modules for new optimizer changes

[connortsui20]

Summary of changes

This PR moves around the modules in the main branch in order to help merge #101 more easily.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 60 lines in your changes missing coverage. Please review.

Project coverage is 88.5%. Comparing base (d0ec48c) to head (29796e9).

Files with missing lines	Patch %	Lines
optd/src/optimizer/hir_cir/into_cir.rs	0.0%	31 Missing ⚠️
optd/src/optimizer/hir_cir/from_cir.rs	0.0%	29 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
optd/src/cir/expressions.rs	0.0% <ø> (ø)
optd/src/cir/plans.rs	0.0% <ø> (ø)
optd/src/cir/rules.rs	0.0% <ø> (ø)
optd/src/optimizer/hir_cir/from_cir.rs	0.0% <0.0%> (ø)
optd/src/optimizer/hir_cir/into_cir.rs	0.0% <0.0%> (ø)

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