Zerofill m_fill for safe fiber recycling, fixes #753

I couldn't find a zerofill algorithm in D so I had to improvise one. This is rarely even important to do b/c D automatically initializes variables as zero-filled, though TaskLocal variables are already initialized when the fiber comes around again.
I think this is usually going to crash. A `TaskLocal` will only be destroyed when the thread has ended. However, in that case the event loop has already been quit and `CoreTask.getThis()` returns `null`. So I'd just remove this.