auto merge of #18967 : aturon/rust/remove-runtime, r=alexcrichton

This PR completes the removal of the runtime system and green-threaded abstractions as part of implementing [RFC 230](https://github.com/rust-lang/rfcs/pull/230).

Specifically:

* It removes the `Runtime` trait, welding the scheduling infrastructure directly to native threads.

* It removes `libgreen` and `libnative` entirely.

* It rewrites `sync::mutex` as a trivial layer on top of native mutexes. Eventually, the two modules will be merged.

* It hides the vast majority of `std::rt`.

This completes the basic task of removing the runtime system (I/O and scheduling) and components that depend on it. 

After this lands, a follow-up PR will pull the `rustrt` crate back into `std`, turn `std::task` into `std::thread` (with API changes to go along with it), and completely cut out the remaining startup/teardown sequence. Other changes, including new [TLS](https://github.com/rust-lang/rfcs/pull/461) and synchronization are in the RFC or pre-RFC phase.

Closes #17325
Closes #18687

[breaking-change]

r? @alexcrichton 

2 files changed, 5 insertions, 5 deletions

diff --git a/src/libstd/sys/common/helper_thread.rs b/src/libstd/sys/common/helper_thread.rs
index 87907fde277..9508d8d9232 100644
--- a/src/libstd/sys/common/helper_thread.rs
+++ b/src/libstd/sys/common/helper_thread.rs
@@ -21,9 +21,9 @@
 //! time.
 
 use mem;
-use rt::bookkeeping;
-use rt::mutex::StaticNativeMutex;
-use rt;
+use rustrt::bookkeeping;
+use rustrt::mutex::StaticNativeMutex;
+use rustrt;
 use cell::UnsafeCell;
 use sys::helper_signal;
 use prelude::*;
@@ -83,7 +83,7 @@ impl<M: Send> Helper<M> {
                     self.lock.lock().signal()
                 });
 
-                rt::at_exit(proc() { self.shutdown() });
+                rustrt::at_exit(proc() { self.shutdown() });
                 *self.initialized.get() = true;
             }
         }
diff --git a/src/libstd/sys/common/net.rs b/src/libstd/sys/common/net.rs
index 9b2b594a9c7..029fc852742 100644
--- a/src/libstd/sys/common/net.rs
+++ b/src/libstd/sys/common/net.rs
@@ -16,7 +16,7 @@ use libc::{mod, c_char, c_int};
 use mem;
 use num::Int;
 use ptr::{mod, null, null_mut};
-use rt::mutex;
+use rustrt::mutex;
 use io::net::ip::{SocketAddr, IpAddr, Ipv4Addr, Ipv6Addr};
 use io::net::addrinfo;
 use io::{IoResult, IoError};