CondSelectGadget for UInts

[nirvantyagi]

This pull request is copied over from arkworks-rs/snark#312

It implements CondSelectGadget for UInt constraint variables. UInt8 is implemented directly, while a macro is included to generate the implementation for UInt16, UInt32, UInt64.

A couple of things for review:

• Is the tracing macro being used correctly? Should any of the inputs be skipped?
• Is the behavior for selecting the value when the inputs have no values correct?

[Pratyush]

Amazing, thanks! Left some comments!

[nirvantyagi]

Suggestions all looked good to me. What does the skip parameter in the tracing macro do? How do you decide which arguments to pass in?

[Pratyush]

By default tracing allows you to record values of variables, but only if they implement Debug. Because for our use case the debug value of a variable doesn't contain useful information anyway, we just skip it.

[Pratyush]

Ahh I think the formatting got messed up, so if you could format then I can merge!

[nirvantyagi]

Oops, looks like my cargo fmt call made some changes the CI didn't like. What should I be doing to format?

[Pratyush]

Sorry just saw this; you want to run cargo +stable fmt.

[nirvantyagi]

Cool, done! Thanks for reviewing!

By the way, I was on nightly because the crate wasn't building on stable -- there was an error with the semver-parser crate.