Add a shared mutability between threads exercise

Closes #3. Thank you @jdm!!! ❇️

threads/threads1.rs

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+// Make this compile! Scroll down for hints :) The idea is the thread
+// spawned on line 17 is completing jobs while the main thread is
+// monitoring progress until 10 jobs are completed. If you see 6 lines
+// of "waiting..." and the program ends without timing out the playground,
+// you've got it :)
+
+use std::sync::Arc;
+use std::thread;
+
+struct JobStatus {
+    jobs_completed: u32,
+}
+
+fn main() {
+    let status = Arc::new(JobStatus { jobs_completed: 0 });
+    let status_shared = status.clone();
+    thread::spawn(move || {
+        for _ in 0..10 {
+            thread::sleep_ms(250);
+            status_shared.jobs_completed += 1;
+        }
+    });
+    while status.jobs_completed < 10 {
+        println!("waiting... ");
+        thread::sleep_ms(500);
+    }
+}
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+// `Arc` is an Atomic Reference Counted pointer that allows safe, shared access
+// to **immutable** data. But we want to *change* the number of `jobs_completed`
+// so we'll need to also use another type that will only allow one thread to
+// mutate the data at a time. Take a look at this section of the book:
+// https://doc.rust-lang.org/stable/book/concurrency.html#safe-shared-mutable-state
+// and keep scrolling if you'd like more hints :)
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+// Do you now have an `Arc` `Mutex` `JobStatus` at the beginning of main? Like:
+// `let status = Arc::new(Mutex::new(JobStatus { jobs_completed: 0 }));`
+// Similar to the code in the example in the book that happens after the text
+// that says "We can use Arc<T> to fix this.". If not, give that a try! If you
+// do and would like more hints, keep scrolling!!
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+// Make sure neither of your threads are holding onto the lock of the mutex
+// while they are sleeping, since this will prevent the other thread from
+// being allowed to get the lock. Locks are automatically released when
+// they go out of scope.
+
+// Ok, so, real talk, this was actually tricky for *me* to do too. And
+// I could see a lot of different problems you might run into, so at this
+// point I'm not sure which one you've hit :) Please see a few possible
+// answers on https://github.com/carols10cents/rustlings/issues/3 --
+// mine is a little more complicated because I decided I wanted to see
+// the number of jobs currently done when I was checking the status.
+
+// Please open an issue if you're still running into a problem that
+// these hints are not helping you with, or if you've looked at the sample
+// answers and don't understand why they work and yours doesn't.
+
+// If you've learned from the sample solutions, I encourage you to come
+// back to this exercise and try it again in a few days to reinforce
+// what you've learned :)