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/not_exported.rs

@@ -0,0 +1,45 @@
+use std::rc::Rc;
+
+use super::{ErrorPosition, ProjectError};
+use crate::representations::location::Location;
+use crate::utils::BoxedIter;
+use crate::{Interner, VName};
+
+/// An import refers to a symbol which exists but is not exported.
+#[derive(Debug)]
+pub struct NotExported {
+  /// The containing file - files are always exported
+  pub file: VName,
+  /// The path leading to the unexported module
+  pub subpath: VName,
+  /// The offending file
+  pub referrer_file: VName,
+  /// The module containing the offending import
+  pub referrer_subpath: VName,
+}
+impl ProjectError for NotExported {
+  fn description(&self) -> &str {
+    "An import refers to a symbol that exists but isn't exported"
+  }
+  fn positions(&self, i: &Interner) -> BoxedIter<ErrorPosition> {
+    Box::new(
+      [
+        ErrorPosition {
+          location: Location::File(Rc::new(i.extern_all(&self.file))),
+          message: Some(format!(
+            "{} isn't exported",
+            i.extern_all(&self.subpath).join("::")
+          )),
+        },
+        ErrorPosition {
+          location: Location::File(Rc::new(i.extern_all(&self.referrer_file))),
+          message: Some(format!(
+            "{} cannot see this symbol",
+            i.extern_all(&self.referrer_subpath).join("::")
+          )),
+        },
+      ]
+      .into_iter(),
+    )
+  }
+}