Change. `zthreads.fork` returns object that support `thread` and `socket` interface

```lua
local actor = zthreads.fork(ctx, function(pipe)
  pipe:send("hello")
  print("thread:", (pipe:recv()))
end):start(true, true)

assert(v == actor)

print("  main:", (actor:recv()))
actor:send("world")

actor:join()
actor:close()
```