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/systems/assertion_error.rs

@@ -0,0 +1,37 @@
+use std::fmt::Display;
+use std::rc::Rc;
+
+use crate::foreign::ExternError;
+use crate::representations::interpreted::ExprInst;
+
+/// Some expectation (usually about the argument types of a function) did not
+/// hold.
+#[derive(Clone)]
+pub struct AssertionError {
+  value: ExprInst,
+  assertion: &'static str,
+}
+
+impl AssertionError {
+  /// Construct, upcast and wrap in a Result that never succeeds for easy
+  /// short-circuiting
+  pub fn fail<T>(
+    value: ExprInst,
+    assertion: &'static str,
+  ) -> Result<T, Rc<dyn ExternError>> {
+    return Err(Self { value, assertion }.into_extern());
+  }
+
+  /// Construct and upcast to [ExternError]
+  pub fn ext(value: ExprInst, assertion: &'static str) -> Rc<dyn ExternError> {
+    return Self { value, assertion }.into_extern();
+  }
+}
+
+impl Display for AssertionError {
+  fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+    write!(f, "Error: {:?} is not {}", self.value, self.assertion)
+  }
+}
+
+impl ExternError for AssertionError {}