Actually, this feature is useful only
when the type of values (the type V_) is a reference type, to avoid using has just
followed by at to get the corresponding value with the very best performances.
Approximation of the actual internal storage capacity.
The capacity will grow automatically
when needed (i.e. capacity is not a limit for the number of values stored). Also note that
the capacity value may not be always accurate depending of the implementation (anyway, this
capacity value is at least equals to count).