Implement some statements

- Impl. expression only statement (expr;)
- Impl. let binding ("let" ident "=" expr;)
- Impl. assignment (ident "=" expr;)

plang2/src/main.rs

@@ -4,7 +4,7 @@ use plang2_lib::*;
 fn main() {
 
     let code = r#"
-        (-(-5+2)*-(2*-sqrt(9))+-(a-6)) % 30
+        a = 54 * sqrt(9);
     "#;
 
     let mut lexer = Lexer::new(code);