MINOR: activity: declare the storage for memory usage statistics

MINOR: activity: declare the storage for memory usage statistics

We'll need to store for each call place, the pointer to the caller
(the return address to be more exact as with free() it's not uncommon
to see tail calls), the number of calls to alloc/free and the total
alloc/free bytes. realloc() will be counted either as alloc or free
depending on the balance of the size before vs after.

We store 1024+1 entries. The first ones are used as hashes and the
last one for collisions.

When profiling is enabled via the CLI, all the stats are reset.