Add allocated_types and allocated_instances

These allow you to get a fast overview of which allocator
type/instance that is causing your memory troubles.
Change average_sizes to be average_block_sizes

When figuring out what sizes to have in your lmbcs, smbcs or sbct it
is the average block size that is interesting. This is because we
usually want to modify the limits to fit as many blocks as we want in
one carrier. The average carrier size is easy enough to figure out by
just looking at the configurations of the allocators.
Fix minor details in sbcs_to_mbcs docs