Update Concurrency course with times (#2007)

As I mentioned in #1536:

* Break into segments at approximately the places @fw-immunant put
breaks
 * Move all of the files into `src/concurrency`
 * Add timings and segment/session metadata so course outlines appear

There's room for more work here, including some additional feedback from
@fw-immunant after the session I observed, but let's do one step at a
time :)

src/concurrency/send-sync/examples.md

@@ -1,3 +1,7 @@
+---
+minutes: 6
+---
+
 # Examples
 
 ## `Send + Sync`

src/concurrency/send-sync/marker-traits.md

@@ -0,0 +1,29 @@
+---
+minutes: 2
+---
+
+# Marker Traits
+
+How does Rust know to forbid shared access across threads? The answer is in two
+traits:
+
+- [`Send`][1]: a type `T` is `Send` if it is safe to move a `T` across a thread
+  boundary.
+- [`Sync`][2]: a type `T` is `Sync` if it is safe to move a `&T` across a thread
+  boundary.
+
+`Send` and `Sync` are [unsafe traits][3]. The compiler will automatically derive
+them for your types as long as they only contain `Send` and `Sync` types. You
+can also implement them manually when you know it is valid.
+
+[1]: https://doc.rust-lang.org/std/marker/trait.Send.html
+[2]: https://doc.rust-lang.org/std/marker/trait.Sync.html
+[3]: ../unsafe/unsafe-traits.md
+
+<details>
+
+- One can think of these traits as markers that the type has certain
+  thread-safety properties.
+- They can be used in the generic constraints as normal traits.
+
+</details>

src/concurrency/send-sync/send.md

@@ -1,3 +1,7 @@
+---
+minutes: 2
+---
+
 # `Send`
 
 > A type `T` is [`Send`][1] if it is safe to move a `T` value to another thread.

src/concurrency/send-sync/sync.md

@@ -1,3 +1,7 @@
+---
+minutes: 2
+---
+
 # `Sync`
 
 > A type `T` is [`Sync`][1] if it is safe to access a `T` value from multiple