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/tcpclient.janet

@@ -0,0 +1,7 @@
+(def conn (net/connect "127.0.0.1" "8000"))
+(printf "Connected to %q!" conn)
+(net/write conn "Echo...")
+(print "Wrote to connection...")
+(def res (net/read conn 1024))
+(pp res)
+(net/close conn)