cl-repl: restructure for future improvements. Replaces temporary handrolled argument parser with external dependency argh.

TODO: Rewrite `argh` in Conlang :^P

cl-repl/src/bin/conlang.rs

@@ -0,0 +1,6 @@
+use cl_repl::cli::run;
+use std::error::Error;
+
+fn main() -> Result<(), Box<dyn Error>> {
+    run(argh::from_env())
+}