Add query module

Extract Query from SelectStatement so that it can be reused for delete
statement.

src/cursor.rs

@@ -305,6 +305,10 @@ impl<'a> BtreeCursor<'a> {
         })
     }
 
+    pub fn is_initialized(&self) -> bool {
+        self.initialized
+    }
+
     /// Move to the specified btree table cell with the key.
     ///
     /// If it does not exist, move to the next cell.

src/expression.rs

@@ -123,6 +123,11 @@ pub enum Expression {
 }
 
 impl Expression {
+    #[inline]
+    pub fn one() -> Self {
+        Self::Const(ConstantValue::Integer(1))
+    }
+
     pub fn from(expr: Expr, table: Option<&Table>) -> Result<Self> {
         match expr {
             Expr::Null => Ok(Self::Null),