Parse var and fn calls in expr

- Parse variable values in expressions
- Parse function calls in expressions

plang2/src/main.rs

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