Add method `is_one` to `One` trait

[cuviper]

From @pyfisch on August 5, 2016 17:15

The Zero trait contains two methods: zero returns the number 0 for the given type and is_zero is provided and can be used to check if a number is 0.

The One trait is similar but contains only the method one to create the number 1. Please also add a method to check if a number is one.

Copied from original issue: rust-num/num#214

[cuviper]

Sure, I'm fine with that. It should be easy if you'd like to add that
yourself and send a PR.

[cuviper]

It turned out not so easy without a breaking change, although we could add it where Self: PartialEq for now to allow a default implementation, and drop that in a breaking change (eventually).

[cuviper]

That was rust-num/num#336 on the next branch.

[clarfonthey]

The issue description/title should be updated to mention removing the PartialEq bound rather than adding the function.

[vks]

Shouldn't this be closed?

[cuviper]

I added the PartialEq note to #47 for breaking changes.