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.

src/core/fiber.c

@@ -337,6 +337,10 @@ JanetFiber *janet_current_fiber(void) {
     return janet_vm_fiber;
 }
 
+JanetFiber *janet_root_fiber(void) {
+    return janet_vm_root_fiber;
+}
+
 /* CFuns */
 
 static Janet cfun_fiber_getenv(int32_t argc, Janet *argv) {