feat: add getter methods on fields of Constraint (#122)

Author: cyx2015s

src/constraint.rs

@@ -29,6 +29,21 @@ impl Constraint {
         self.name = Some(name);
         self
     }
+
+    /// The expression that is constrained to be null or negative
+    pub fn expression(&self) -> &Expression {
+        &self.expression
+    }
+
+    /// if is_equality, represents expression == 0, otherwise, expression <= 0
+    pub fn is_equality(&self) -> bool {
+        self.is_equality
+    }
+
+    /// get the constraint name, if it exists.
+    pub fn name(&self) -> Option<&str> {
+        self.name.as_deref()
+    }
 }
 
 impl FormatWithVars for Constraint {