Export num_traits dependency as a public module

[Dushistov]

I want to use your crate, but do not want to manualy add dependicies of your crate.
As suggested https://stackoverflow.com/questions/44876113/how-do-i-use-a-crate-from-another-crate-without-explicitly-defining-a-new-depend here I suggest export num_traits.

[cardoe]

You really shouldn't need that. You should be able to use enum_primitive::FromPrimitive; in your code and that's it. You don't need to ever touch num_traits.

[EkremDincel]

@cardoe Somebody should add this information to the crates.io page, the example here is not working in 1.49 but use enum_primitive::FromPrimitive; works.

[cardoe]

@EkremDincel Unfortunately this crate is mostly dead. I've written the derive for it as a separate project and I've asked the author to let me take it over so I could merge the two and clean everything up but I've never heard back.