refactor: tidy; fix runtime path resolution

Justfile

@@ -0,0 +1,15 @@
+[no-exit-message]
+@repl:
+  cargo run --bin repl
+
+[no-exit-message]
+@eval expr:
+  cargo run --bin eval -- '{{expr}}'
+
+[no-exit-message]
+@replr:
+  cargo run --bin repl --release
+
+[no-exit-message]
+@evalr expr:
+  cargo run --bin eval --release -- '{{expr}}'