feat(validators): unused param warning

Params being unused were being incorrectly reported.
This was because params need to be initialized
at a scope above both the validator functions. This
manifested when using a multi-validator where one of
the params was not used in both validators.

The easy fix was to add a field called
`is_validator_param` to `ArgName`. Then
when infering a function we don't initialize args
that are validator params. We now handle this
in a scope that is created before in the match branch for
validator in the `infer_definition` function. In there
we call `.in_new_scope` and initialize params for usage
detection.

crates/aiken-lang/src/tests/check.rs

@@ -113,6 +113,47 @@ fn validator_in_lib_warning() {
     ))
 }
 
+#[test]
+fn multi_validator() {
+    let source_code = r#"
+      validator(foo: ByteArray, bar: Int) {
+        fn spend(_d, _r, _c) {
+          foo == #"aabb"
+        }
+
+        fn mint(_r, _c) {
+          bar == 0
+        }
+      }
+    "#;
+
+    let (warnings, _) = check_validator(parse(source_code)).unwrap();
+
+    assert_eq!(warnings.len(), 0)
+}
+
+#[test]
+fn multi_validator_warning() {
+    let source_code = r#"
+      validator(foo: ByteArray, bar: Int) {
+        fn spend(_d, _r, _c) {
+          foo == #"aabb"
+        }
+
+        fn mint(_r, _c) {
+          True
+        }
+      }
+    "#;
+
+    let (warnings, _) = check_validator(parse(source_code)).unwrap();
+
+    assert!(matches!(
+        warnings[0],
+        Warning::UnusedVariable { ref name, .. } if name == "bar"
+    ))
+}
+
 #[test]
 fn if_scoping() {
     let source_code = r#"