treewide: Replace <name>_cnt by n_<name>s and <name>_cap by allocated_<name>.

treewide: Replace <name>_cnt by n_<name>s and <name>_cap by allocated_<name>.

I used to think that _cnt was a good suffix for a count representing the
number of items in an array, but I haven't used that convention in a long
time and it looks wrong now.  This changes these to an n_ prefix instead.

Similarly, but in far fewer cases, replace <name>_cap for the allocated
capacity of an array by allocated_<name>.

This doesn't change the name of ref_cnt members used as reference counts.
Those are pure counts, not the length of anything.

This doesn't change the Perl library because that's an external API.