Add an example of using a function-like proc macro in expression position

A simple example:

```rust
use proc_macro::TokenStream;

#[proc_macro]
fn comptime(_input: TokenStream) -> TokenStream {
    "1 + 2".parse().unwrap()
}

fn main() {
    println!("1 + 2 = {}", comptime!());
}
```

This particular example isn't particularly interesting, but I don't know proc macros very well yet :)