Helper function for getting/setting cache data

[johnjamesjacoby]

Most of the time when we want something from the object cache we go through a routine of: wp_cache_get(); checking a result; setting a default; setting the cache with wp_cache_set(); returning. Core should offer a neat way to do this all in one fell-swoop.

Attached patch is a proposed solution.

I've never needed to set a default when a cache value is not present.

Could you provide a usage example for the new function?

I'm particularly interested in how the callback parameter would work, knowing that generating a value to be cached would require various bits of information from the current context.