num: remove pub from sub-crates

[cuviper]

Rust 1.4 through 1.7 will issue warnings that pub extern crate does
not work as expected, although it appears to be fine. But Rust 1.8 and
later issue #[warn(private_in_public)] for pub use num_foo as foo if
that crate isn't public, and that's headed toward a hard error.

@bluss suggested instead pub mod foo { pub use num_foo::*; }, which
I thought I had tried before, but it appears to work fine on all
versions of Rust. Let's do it!

A small downside is that docs for num::foo now just show the wildcard
reexport, instead of direct documentation, but at least there's a link
to follow to the sub-crate.

It's a breaking change that num::num_foo paths are no longer public,
but we didn't really want those exposed in the first place. I consider
this minor -- people should either use the num::foo module as before
the split-up, or use the num_foo crate directly.

Fixes #189.

[bluss]

I only mentioned it exists, don't know if it will work well.

[cuviper]

That's partly why I opened a PR instead of pushing this right away. It looks fine so far.

[cuviper]

@homu r+

[homu]

📌 Commit 865c491 has been approved by cuviper

[hauleth]

In Octavo I am using pub extern crate name as other_name; which has that advantage that this provide better documentation.

[cuviper]

Sure, that would be nice, but it runs afoul of the same warning as #189 in older rustc.