Issue Description:

In its current implementation, Set and Dictionary will never reduce their storage size. This can cause increased memory consumption for long-living Set/Dictionary values.

Change Set and Dictionary to shrink storage after removals if the load factor goes below some predefined minimum.

• The (amortized) asymptotic performance of removals must be the same before/after this change.

• Storage must never shrink below the capacity reserved by the user. This means that Set and Dictionary instances must start keeping track of any reserved capacity.

This change can be tested by looking at the capacity of the Set/Dictionary after removals.