Add simple and very rough interpreter

- Implemented an interpreter that
  - Only works with int64 values
  - Executes maths expressions
  - Declares and assigns variables in global table
  - Can use variables in expressions
  - Debug prints expression only statements
  - Can NOT use, or declare functions

plang2/src/main.rs

@@ -4,9 +4,9 @@ use plang2_lib::*;
 fn main() {
 
     let code = r#"
-        a = 54 * 3;
-        b = 5;
-        print("{}", a + b);
+        let a = 5 * 3;
+        let b = 5;
+        (a + b * 3) / 5;
     "#;
 
     let mut lexer = Lexer::new(code);
@@ -16,8 +16,12 @@ fn main() {
     println!("Tokens: \n{}\n", tokens);
 
     let mut parser = Parser::new(tokens);
-    let expr = parser.parse().unwrap();
+    let prog = parser.parse().unwrap();
 
-    println!("{:#?}", expr);
+    println!("{:#?}\n\n", prog);
+
+    let mut interpreter = Interpreter::new(prog);
+
+    interpreter.run();
 
 }