Add preliminary channel implementation.

examples/channel.janet

@@ -0,0 +1,15 @@
+(def c (ev/chan 4))
+
+(defn writer []
+  (for i 0 10
+    (ev/sleep 0.1)
+    (print "writer giving item " i "...")
+    (ev/give c (string "item " i))))
+
+(defn reader [name]
+  (forever
+    (print "reader " name " got " (ev/take c))))
+
+(ev/call writer)
+(each letter [:a :b :c :d :e :f :g]
+  (ev/call reader letter))