Add ev/select and ev/rselect initial implementation.

Getting closer to a CSP implmententation. Probably
useful to move scheduling fields outside of fibers
and into an external table.

examples/select.janet

@@ -0,0 +1,22 @@
+(def channels
+  (seq [:repeat 5] (ev/chan 4)))
+
+(defn writer [c]
+  (for i 0 3
+    (ev/sleep 0.1)
+    (print "writer giving item " i " to " c "...")
+    (ev/give c (string "item " i)))
+  (print "Done!"))
+
+(defn reader [name]
+  (forever
+    (def c (ev/select ;channels))
+    (print "reader " name " got " (ev/take c) " from " c)))
+
+# Readers
+(each letter [:a :b :c :d :e :f :g]
+  (ev/call reader letter))
+
+# Writers
+(each c channels
+  (ev/call writer c))