Runtime removal: refactor process

This patch continues the runtime removal by moving and refactoring the
process implementation into the new `sys` module.

Because this eliminates APIs in `libnative` and `librustrt`, it is a:

[breaking-change]

This functionality is likely to be available publicly, in some form,
from `std` in the future.

src/libstd/sys/windows/mod.rs

@@ -40,6 +40,7 @@ pub mod tcp;
 pub mod udp;
 pub mod pipe;
 pub mod helper_signal;
+pub mod process;
 
 pub mod addrinfo {
     pub use sys_common::net::get_host_addresses;