hw: move double list to extra header
hw: fix type casting in Double_list
hw: make Double_list_item::listed public
hw: implement Double_list::for_each
hw: helping semantics in the form of Helping_node
hw: rename Thread::State SCHEDULED in ACTIVE

As soon as helping is used a thread may also be in a blocking state when its
scheduling context is scheduled. Hence, the state designation SCHEDULED for
an active thread would be misleading.
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