Cleaned up atoms

- Atoms now use MFBI to distinguish between thin and owned atoms.
- Introduced TryFromExpr and ToExpr (formerly ToClause) from the old FFI
- Standardized on Bottom being a ProjErr, which means that there will be no RTErr

orchid-extension/src/lib.rs

@@ -10,4 +10,6 @@ pub mod other_system;
 pub mod system;
 pub mod system_ctor;
 pub mod tree;
-pub mod try_from_expr;
+pub mod conv;
+pub mod atom_thin;
+pub mod atom_owned;