Add feature to query number of allocations

[Robbepop]

This is concerning the jemalloc-ctl crate:

It is often important to distinguish between the number of allocations and not only the number of allocated bytes.
For example it could make a big difference if an application allocated the same amount of bytes in one big chunk or in many tiny pieces.

It would be very nice if we could query the number of allocations done by the underlying application if that's possible to add. In the same vein it might also be interesting to add a way to query the number of memory frees.

I am not sure at this point but it might be that the required API is missing from jemalloc:
jemalloc/jemalloc#1878