refactor(core): abstract memo table into a trait (#211)

The current memo table implementation is refactored into a trait, and
the implementation is called `NaiveMemo`.

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Signed-off-by: Alex Chi <[email protected]>

Author: skyzh

Diff for: optd-core/src/cascades.rs

@@ -4,6 +4,6 @@ mod memo;
 mod optimizer;
 mod tasks;
 
-pub use memo::Memo;
+pub use memo::{Memo, NaiveMemo};
 pub use optimizer::{CascadesOptimizer, ExprId, GroupId, OptimizerProperties, RelNodeContext};
 use tasks::Task;