Refactor the standard library into a crate of crates

[brson]

It is very desirable for std to be maximally portable to a variety of enviroments, and I am slightly worried that if we don't focus on making it well-factored now, before 1.0, that portability will be hampered later.

Sub-dividing std into crates to isolate platform-dependent parts will establish clear dependencies within the library. We can preserve std's interface by reexporting.

I can imagine at least the following subdivision:

• libc
• win - the parts of std::libc that are actually the windows api
• prim - the primitive types
• path
• os
• comm
• task
• rt
• std

[brson]

This will probably require some clever thought about how to deal with failure and other task-local services in the lowest-level crates, since these are plattform-specific concepts.

[alexcrichton]

liblibc

This sounds like a fantastic to me. A "compilation profile" would sound to me like a subset of crates instead of littering #[cfg(foo)] everywhere (perhaps)

[brson]

I have a patch that factors a few primitive things out of std.

[alexcrichton]

Closing in favor of #13851