refactor: use lib.rs in main.rs

this avoids duplicating unit tests, for instance

https://stackoverflow.com/a/77038955

src/main.rs

@@ -4,8 +4,7 @@ use std::{
     process::ExitCode,
 };
 
-mod formatting;
-mod nodes;
+use tylc as lib;
 
 fn main() -> ExitCode {
     let input = match env::args().skip(1).rev().reduce(|x, a| a + &x) {
@@ -17,7 +16,7 @@ fn main() -> ExitCode {
         }
     };
 
-    match formatting::pretty_eval(input, 10) {
+    match lib::formatting::pretty_eval(input, 10) {
         Ok(s) => println!("{s}"),
 
         Err(e) => {