Working TCP echo server and client.

Required a few changes to APIs, namely janet_root_fiber()
to get topmost fiber that is active in the current scheduler.
This is distinct from janet_current_fiber(), which gets the bottom
most fiber in the fiber stack - it might have a parent, and so cannot
be reliably resumed.
This is the kind of situation that makes symmetric coroutines more
attractive.

examples/tcpserver.janet

@@ -0,0 +1,13 @@
+(defn handler
+  "Simple handler for connections."
+  [stream]
+  (def id (gensym))
+  (def b @"")
+  (print "Connection " id "!")
+  (while (net/read stream 1024 b)
+    (net/write stream b)
+    (buffer/clear b))
+  (printf "Done %v!" id)
+  (net/close stream))
+
+(net/server "127.0.0.1" "8000" handler)