Improve Borrow Checker to Handle Scopes and Borrow Checking

[notJoon]

1. Handle multiple immutable borrows: The BorrowChecker now allows multiple immutable borrows of the same variable, aligning with Rust's borrow rules. In short, no longer returns an error when a variable is immutably borrowed multiple times.

2. Handle scopes correctly: The BorrowChecker now correctly handles variables declared in outer scopes when checking inner scopes. It uses a stack of scopes, each represented as a hashmap. When entering a new scope, it pushes a new hashmap onto the stack, and when leaving a scope, it pops the hashmap off the stack.

3. Add more test cases