... and that doesn't necessarily mean that malloc() failed.
We do *not* want to evict a heavy-hitter by an unfrequent element.

Note: even the implementation currently in master *did* return NULL,
so some parts of the code were just wrongly returning ENOMEM.

The implementation is now similar to set-associative caches
that x86 CPU use.  Also the API is changed a bit, leading to
slight simplification of our use patterns.

fixes potential cache key oob write

the api is still a global variable, but it would be the
best if the namedb_t carried the pointer to the api,
so it needs to change in libknot later

interface split to ‘map’ and ‘set’
added documentation
array: added array_tail() operation