Macro wlambda::set_vval_method

macro_rules! set_vval_method {
    ($vv: expr, $obj: ident, $method: tt, $min: expr, $max: expr, $env: ident, $argc: ident, $b: block) => { ... };
}

This macro helps with exporting something with a more OO interface instead of using a VValUserData trait implementation.

 use wlambda::*;
 use wlambda::set_vval_method;

 let mut ctx = EvalContext::new_default();

 let some_ref = std::rc::Rc::new(std::cell::RefCell::new(10));
 let obj = VVal::map();

 set_vval_method!(obj, some_ref, get_it, None, None, _env, _argc, {
     Ok(VVal::Int(*some_ref.borrow()))
 });

 ctx.set_global_var("I", &obj);

 assert_eq!(ctx.eval("I.get_it[]").unwrap().s(), "10");
 *some_ref.borrow_mut() += 11;
 assert_eq!(ctx.eval("I.get_it[]").unwrap().s(), "21");