Add thread example.

Also remove reference to pthread_t in the JanetThread structure.

examples/threads.janet

@@ -0,0 +1,18 @@
+(defn worker-main
+  [parent]
+  (def name (thread/receive parent))
+  (for i 0 10
+    (os/sleep 1)
+    (print "thread " name " wakeup no. " i))
+  (thread/send parent :done))
+
+(defn make-worker
+  [name]
+  (-> (thread/new make-image-dict) (thread/send worker-main) (thread/send name)))
+
+(def bob (make-worker "bob"))
+(os/sleep 0.5)
+(def joe (make-worker "joe"))
+
+(thread/receive bob)
+(thread/receive joe)