Auto merge of #5147 - Eh2406:restucture, r=alexcrichton

restructure `Activations` for better clone

This builds on the work in #5121 When we last met we had:
5000000 ticks in ~48 sec, 0r 104k ticks/sec
This small change brings us to:
5000000 ticks in ~21 sec, 0r 238k ticks/sec
Edit: sorry for the large diff only the last commit is new. The rest are from #5121