Parse multiple statements

- parse() now parses a Vec<Statement> that can represent basically a
  full program
- Now multiple lines / statements are parsed together

plang2/src/main.rs

@@ -4,7 +4,9 @@ use plang2_lib::*;
 fn main() {
 
     let code = r#"
-        a = 54 * sqrt(9);
+        a = 54 * 3;
+        b = 5;
+        print("{}", a + b);
     "#;
 
     let mut lexer = Lexer::new(code);