Return true/false for promote & demote

[levkk]

I want promote/demote to return false if the key doesn't exist. A common pattern for me is:

if cache.contains(&key) {
     cache.promote(&key);
}

This requires to check the map twice, which for large keys, is noticeable performance-wise.

Now I can just do:

if cache.promote(&key) {
     // ...
}

[jeromefroe]

Thanks for the contribution @levkk!