Implement better capacity in our group structures

[yoshuawuyts]

#168 fixes the immediate insertion time issues of FutureGroup::insert - however the code is, how should I say this, not great?

We need to implement our own exponential capacity algorithm. This means implementing a reserve method which ensures there is enough capacity. And then making sure that if we run out of capacity we always double our space. We may even want to rename some of our existing resize methods to reserve, since they don't take items where Vec::resize does.

This probably needs to be an immediate follow-up to #168; I don't think we should release a new version without it.

[yoshuawuyts]

Porting my comment from #175:

@matheus-consoli are you still ok to file that PR? If not, feel free to re-assign this to me!

[matheus-consoli]

yes! sorry for the delay, busy week!