Future: support for multiple callbacks

tests/async/tcallbacks.nim

@@ -0,0 +1,20 @@
+discard """
+  exitcode: 0
+  output: '''3
+2
+1
+5
+'''
+"""
+import asyncfutures
+
+let f1: Future[int] = newFuture[int]()
+f1.addCallback(proc() = echo 1)
+f1.addCallback(proc() = echo 2)
+f1.addCallback(proc() = echo 3)
+f1.complete(10)
+
+let f2: Future[int] = newFuture[int]()
+f2.addCallback(proc() = echo 4)
+f2.callback = proc() = echo 5
+f2.complete(10)