Add an "async" session (#496)

* beginning of an Async section

* address review comments

* Add futures page (#497)

NOTE: `mdbook test` does not allow code samples to reference other crates, so
they must be marked as `compile_fail`; see #175.

* Add Runtimes & Tasks (#522)

These concepts are closely related, and there's not much else to know
about runtimes other than "they exist".

This removes the bit about futures being "inert" because it doesn't
really lead anywhere.

* Async chapter (#524)

* Add async channels chapter

* Async control flow

* Async pitfalls

* Separate in multiple chapters + add daemon section

* Merge reentering threads in blocking-executor

* async_trait

* Async fixes (#546)

* Async: some ideas for simplifying the content (#550)

* Simplify the async-await slide
* Shorten futures and move it up
* Add a page on Tokio

* Modifications to the async section (#556)

* Modifications to the async section

* Remove the "Daemon" slide, as it largely duplicates the "Tasks" slide.
  The introduction to the "Control Flow" section mentions tasks as a
  kind of control flow.

* Reorganize the structure in SUMMARY.md to correspond to the directory
  structure.

* Simplify the "Pin" and "Blocking the Executor" slides with steps in
  the speaker notes to demonstrate / fix the issues.

* Rename "join_all" to "Join".

* Simplify some code samples to shorten them, and to print output rather
  than asserting.

* Clarify speaker notes and include more "Try.." suggestions.

* Be consistent about where `async` blocks are introduced (in the
  "Tasks" slide).

* Explain `join` and `select` in prose.

* Fix formatting of section-header slides.

* Add a note on async trait (#558)

---------

Co-authored-by: sakex <alexandre@senges.ch>
Co-authored-by: rbehjati <razieh@google.com>

src/exercises/day-4/async.md

@@ -0,0 +1,3 @@
+# Exercises
+
+TBD