feat: initial rule engine + memo table (#22)

## Problem

The persistent memo layer and the rule engine are the core components
that facilitate query plan transformation.

Having an initial setup is important for people to work on the same
codebase simultaneously.

## Summary of changes

- A memo table implementation backed by SQLite.
- Low-level IR for the rule engine proof concept.

---------

Signed-off-by: Yuchen Liang <[email protected]>
Co-authored-by: Yuchen Liang <[email protected]>
Co-authored-by: Yuchen Liang <[email protected]>

Author: AlSchlo

.gitignore

@@ -7,3 +7,8 @@ target/
 
 # MSVC Windows builds of rustc generate these, which store debugging information
 *.pdb
+
+.env
+*.db
+*.db-shm
+*.db-wal