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/unexpected_directory.rs

@@ -0,0 +1,27 @@
+use super::{ErrorPosition, ProjectError};
+use crate::utils::iter::box_once;
+use crate::utils::BoxedIter;
+use crate::{Interner, VName};
+
+/// Produced when a stage that deals specifically with code encounters
+/// a path that refers to a directory
+#[derive(Debug)]
+pub struct UnexpectedDirectory {
+  /// Path to the offending collection
+  pub path: VName,
+}
+impl ProjectError for UnexpectedDirectory {
+  fn description(&self) -> &str {
+    "A stage that deals specifically with code encountered a path that refers \
+     to a directory"
+  }
+  fn positions(&self, i: &Interner) -> BoxedIter<ErrorPosition> {
+    box_once(ErrorPosition::just_file(i.extern_all(&self.path)))
+  }
+  fn message(&self, i: &Interner) -> String {
+    format!(
+      "{} was expected to be a file but a directory was found",
+      i.extern_all(&self.path).join("/")
+    )
+  }
+}