This commit adds the boilerplate files and fixes the CI workflows.

This commit adds a first draft of a memo table trait and a persistent
memo table implementation backed by SeaORM entities.

This commit adds the `src/expression` module which contains a very
simple representation of Cascades expressions.

The `Memo` trait interface and implemenation has also changed, where
it now correctly detects exact match duplicates, and it does not track
fingerprints for physical expressions (only logical).

TODO: Add more tests.
TODO: Figure out how to test in CI.

This commit completely refactors the memo table, removing the `Memo`
trait and instead placing all methods directly on the `PersistentMemo`
structure itself.

This also cleans up some code in other places.

This commit replaces the specific expression types with traits that
define the behavior the in-memory represenations of both logical and
physical expressions need to have. Right now, the `PhysicalExpression`
trait does not do that much, but the `LogicalExpression` trait is super
important to how the persistent memo table works.

WIP