Make at_exit initialize lazily
Remove rt::bookkeeping

This commit removes the runtime bookkeeping previously used to ensure
that all Rust tasks were joined before the runtime was shut down.

This functionality will be replaced by an RAII style `Thread` API, that
will also offer a detached mode.

Since this changes the semantics of shutdown, it is a:

[breaking-change]
Remove rt::{local, local_data, thread_local_storage}
Remove rt::{mutex, exclusive}
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]

Closes #18000
Add blocking support module for channels
Revise rt::unwind
Fallout from new thread API
Disable capture_stderr test for now