Export all symbols at crate root as in libc

[kamalmarhubi]

As part of implementing RFC 1291, the structure of the libc crate was changed. It now exports all symbols at the top level. I think it's worth considering the same structure for nix.

[Susurrus]

I don't think this makes sense, primarily from a documentation point of view. It's nice being able to provide docs for entire modules for some of the wrapper types that nix implements. It also makes it easier for people from from C to find the man pages. I think it made sense for libc to do this given that they don't care about documentation as they're such a low-level dependency, but for nix we want to have a really good documentation story that includes being able to reuse all the existing docs on libc stuff.

I'm closing this as a WONTFIX because of the above.