Add 'again' keyword for recursive calls

The `again` keyword is introduced to facilitate explicit recursive
function calls.
It is restricted to tail-call positions to prevent dead code and ensure
TCO
optimization. Type checking is enhanced to validate argument types
against
function parameters.

src/ast/types.rs

@@ -249,6 +249,16 @@ impl StaticType {
                 }
                 elements.iter().all(|e| e.is_assignable_from(inner))
             }
+            // Tuple to Tuple (Element-wise)
+            (StaticType::Tuple(elements), StaticType::Tuple(other_elements)) => {
+                if elements.len() != other_elements.len() {
+                    return false;
+                }
+                elements
+                    .iter()
+                    .zip(other_elements.iter())
+                    .all(|(e, o)| e.is_assignable_from(o))
+            }
             // A Matrix is a Vector (of Vectors/Matrices)
             (StaticType::Vector(inner, len), StaticType::Matrix(m_inner, shape)) => {
                 if shape.is_empty() || shape[0] != *len {