Transform NimGo into a full feature multithreaded library

[Alogani]

Motivations: This discussion has been started on nim's forum with vigorous examples: https://forum.nim-lang.org/t/11720. It results that transforming NimGo into a multithreaded library like goroutines is possible and desirable.

Definitions

Those definitions are important to agree on what we do and how.

• "Thread" = Os threads
• Task = any block of code that could be suspended and resumed
• Events: This term can regroup
  • I/O events (std/selectors)
  • timers (internal heapqueue)
  • callbacks (tasks resumed by another task) and cancellation
• EventLoop = loop in charge of checking which events are available and inform about the tasks associated with them
• DispatcherLoop = loop in charge of distributing tasks between threads
  (EventLoop and DispatcherLoop may or may not be the same loop, generally the EventLoop fills the DispatcherLoop)

Roadmap

This posts will link to all tasks needed to achieve this (the issue/PR will be linked here for each one as they are created):

• Rename goAsync keyword #18
• Transform Coroutine Ref to a SharedPtr
• Remove the usage of SafeContainer
• Find a design for the Task Dispatcher Open reflexions to implement a M:N multi threaded dispatcher on NimGo #21
• Implements a kind of locks/semaphore that instead of blocking will put current's coroutine at the end of the queue ?
• [...]