Proc macro based version

Do you have a plan to rebuild the macros based on the [proc_macro](https://github.com/rust-lang/rfcs/blob/master/text/1566-proc-macros.md)?

Those are sweeter.
```rust
#[cached(Fib)]
fn fib(n: u64) -> u64 {
     if n == 0 || n == 1 { 
        return n; 
      }
      fib(n-1) + fib(n-2)
}
```