Always cache config values

[HashNotAdam]

When retrieving a value from Dry::Configurable::Config#[], the value is only cached to the _values array if it is considered "cloneable". Unfortunately, checking for cloneability is a costly exercise (at least when called at the frequency of Dry::View).

The code states that this occurs as a means of helping understand when a setting has been mutated; however, I would like to suggest that the increase in performance that can be achieved in Dry::View rendering is more valuable. Saying this, I suggest modifying Dry::View rather than Dry::Configurable::Config because I doubt most uses have the same performance requirements.

In my benchmarks, I'm using a highly contrived template that loads 100 nested templates that do nothing more than print out their name as well a couple of random numbers to ensure values aren't being cached so, while it is designed somewhat to replicate a site with many components, it's far from true to life. Saying this, my tests have found this has given me a roughly 70% performance increase:

Calculating -------------------------------------
              Before    100.263  (±12.0%) i/s -      1.008k in  10.076792s
               After    169.272  (± 7.1%) i/s -      1.696k in  10.076434s

It's tricky to solve this elegantly so I'm very open to discussing alternative approaches.