Most files suffered major changes

- Less ambiguous syntax
- Better parser (Chumsky only does tokenization now)
- Tidy(|ier) error handling
- Facade for simplified embedding
- External code grouped in (fairly) self-contained Systems
- Dynamic action dispatch
- Many STL additions

src/error/visibility_mismatch.rs

@@ -0,0 +1,30 @@
+use std::rc::Rc;
+
+use super::project_error::{ErrorPosition, ProjectError};
+use crate::representations::location::Location;
+use crate::utils::iter::box_once;
+use crate::utils::BoxedIter;
+use crate::{Interner, VName};
+
+/// Multiple occurences of the same namespace with different visibility
+#[derive(Debug)]
+pub struct VisibilityMismatch {
+  /// The namespace with ambiguous visibility
+  pub namespace: VName,
+  /// The file containing the namespace
+  pub file: Rc<Vec<String>>,
+}
+impl ProjectError for VisibilityMismatch {
+  fn description(&self) -> &str {
+    "Some occurences of a namespace are exported but others are not"
+  }
+  fn positions(&self, i: &Interner) -> BoxedIter<ErrorPosition> {
+    box_once(ErrorPosition {
+      location: Location::File(self.file.clone()),
+      message: Some(format!(
+        "{} is opened multiple times with different visibilities",
+        i.extern_all(&self.namespace).join("::")
+      )),
+    })
+  }
+}