Uint: `const fn` comparisons are `pub(crate)`

I was recently trying to add some `const fn` friendly checks that a field element doesn't overflow the modulus, and while `Uint::lt` would do what I want, it's currently `pub(crate)`, as are other comparisons like `const fn eq` and `const fn gt`.

@fjarri @andrewwhitehead is there a particular reasons these methods aren't exposed? They seem useful