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