Merge changes I17ccda26,Ie4904c38,I93b83535,I595819d0,Id2975243, ...

* changes:
  use on_XYZ as convention for event callbacks inside HciLayer::impl
  Clean up namespaces inside hci_layer
  Collapse send_acl into dequeue_and_send_acl
  Move hal callbacks into their own object, to clarify execution context
  Add function on EventHandler<T> to post events back
  Make EventHandler generic, so we use it to replace SubeventHandler
  Add common::BindOn to simplify binding callbacks to instance methods.
  Begin migrating context switching out of impl
  Collapse HciLayer::GetAclQueueEnd()
  Make hci_layer module lifecycle functions protected, to match module.h
  Move trivial functions from hci_layer.cc to hci_layer.h
  Add CallOn variant of Module.Call()
  Add Module.Call() to cut down on module boilerplate