A taste of what const generics could enable in the future

```rust

![feature(genericconstexprs)]

use genericpredicates::genericpredicates;

generic_predicates! { pub fn foo() where (N > 23, "N must be greater than 23") {

}

}

fn main() { // This compiles. foo::<24>();

// This doesn't.
foo::<23>();

} ```