Return CacheResult from Client#get_cas

[drinkbeer]

Summary

• extend Dalli::CacheResult with an optional cas_token
• return a frozen CacheResult from no-block Client#get_cas, including hit, miss, and stale state
• preserve the existing block yield contract and the behavior of cas, cas!, set_cas, and get_multi_cas

This intentionally changes no-block get_cas from [value, cas_token] to a result object. Callers should use result.value and result.cas_token; a true miss has token 0, while non-CAS status reads leave the token nil.

Related (Shopify internal): https://github.com/shop/issues-caching-platform/issues/303

Validation

• bundle exec rake (458 runs, 50,356 assertions)
• bundle exec rubocop --parallel