Save permissions structure to Rails.cache if a cache key is passed

[dansingerman]

With a large roles.yml, even if you are using a UserPermissionCachingDecorator to intercept db access (or using #20), the calls to permit can be pretty slow.

In an app with ~200 entries in the roles.yml it was taking ~130ms to process all the permit calls.

This change allows a cache_key parameter to be passed in, and if it is, the structure that is passed to CanCanCan is cached by rails, saving most of the processing time.

Note this should not be a replacement for #20 - which is caching at a different level and should also be implemented.

[adampal]

@dansingerman sorry I'm a bit late to the party on this one.
The change looks good. I just wanted to ask your thoughts on using OpenStruct rather than a Hash. Given this is a performance optimisation, a Hash should be much faster than an OpenStruct. The ruby lang guide (here - https://docs.ruby-lang.org/en/master/OpenStruct.html) has this:

Creating an open struct from a small Hash and accessing a few of the entries can be 200 times slower than accessing the hash directly.

Given this is just internal, my thinking is we'd be better off just using a Hash and missing out on the syntactic sugar provided by OpenStruct in favour of performance.

[dansingerman]

@adampal I benchmarked OpenStructs vs Hashes and you're totally right - so I've raised a PR here: #25