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

[tarcieri]

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