Introduce std::thread

Also removes:

* `std::task`
* `std::rt::task`
* `std::rt::thread`

This commit completely changes the thread model (threads are detached by
default) and API (including park/unpark for blocking). It is a:

[breaking-change]
Add blocking support module for channels
Remove rt::{mutex, exclusive}
Revamp channels to use new blocking support
Remove sync/lock.rs
Revise rt::unwind
Fallout from new thread API