Add data for nonzero_is_power_of_two

[jdkaplan]

Add data for nonzero_is_power_of_two, which defines is_power_of_two on nonzero unsigned integer types.

Decisions I made that I'm not totally sure about (but would be happy to fix in this PR):

1. Put this data under unstable. Data to add #16 has this listed as "stable", but usage still requires nightly Rust and #![feature].
2. Leave doc_path empty. There are docs for each type (for example, https://doc.rust-lang.org/std/num/struct.NonZeroUsize.html#method.is_power_of_two), but picking just one didn't seem right. The most similar feature (data/1.40/const_is_power_of_two.md) also doesn't set a doc_path.

[jplatte]

Both decisions make sense. That it's listed under stable is a mistake. And doc_path is really more meant for language features that you can't just use the rustdoc search for.

[jplatte]

Other than the title, looks good. Thanks!