Fix incoherent 'UnknownVariable' being returned in type-check

I initially removed the 'UnkownTypeConstructor' since it wasn't used anywhere and was in fact dead-code. On second thoughts however, it is nicer to provide a slightly better error message when a constructor is missing as well as some valid suggestion. Prior to that commit, we would simply return a 'UnknownVariable' and the hint might suggest lowercase identifiers; which is wrong.

crates/aiken-lang/src/tipo/environment.rs

@@ -309,10 +309,13 @@ impl<'a> Environment<'a> {
         location: Span,
     ) -> Result<&ValueConstructor, Error> {
         match module {
-            None => self.scope.get(name).ok_or_else(|| Error::UnknownVariable {
+            None => self
+                .scope
+                .get(name)
+                .ok_or_else(|| Error::UnknownTypeConstructor {
                     location,
                     name: name.to_string(),
-                variables: self.local_value_names(),
+                    constructors: self.local_constructor_names(),
                 }),
 
             Some(m) => {
@@ -577,6 +580,14 @@ impl<'a> Environment<'a> {
             .collect()
     }
 
+    pub fn local_constructor_names(&self) -> Vec<String> {
+        self.scope
+            .keys()
+            .filter(|&t| t.chars().next().unwrap_or_default().is_uppercase())
+            .map(|t| t.to_string())
+            .collect()
+    }
+
     fn make_type_vars(
         &mut self,
         args: &[String],

crates/aiken-lang/src/tipo/error.rs

@@ -799,7 +799,7 @@ Perhaps, try the following:
         suggest_neighbor(name, constructors.iter(), "Did you forget to import it?")
     ))]
     UnknownTypeConstructor {
-        #[label]
+        #[label("unknown constructor")]
         location: Span,
         name: String,
         constructors: Vec<String>,