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/mod.rs

@@ -0,0 +1,13 @@
+//! Constants exposed to usercode by the interpreter
+mod assertion_error;
+mod asynch;
+pub mod cast_exprinst;
+pub mod codegen;
+mod io;
+mod runtime_error;
+pub mod stl;
+
+pub use assertion_error::AssertionError;
+pub use asynch::{AsynchConfig, InfiniteBlock, MessagePort};
+pub use io::{io_system, IOStream, IOSystem};
+pub use runtime_error::RuntimeError;