CacheStorage

[maxg-square]

2/3 for caching changes.
Big-picture PR here: https://github.com/square/Blueprint/pull/574/files

• Adds CacheStorage, an object which hangs off of Environment. CacheStorage has a similar API to Environment, with consumers declaring their types and default values by declaring a CacheKey.
• Adds internal values to Environment which do not participate in ContextuallyEquivalent checks.
• Adds FrozenEnvironment, which is a snapshot of an Environment that holds a subset of its storage, and is used by ValidatingCache.
• Adds ValidatingCache, which allows a key to be set with a value that's associated with a validation function.
• Adds EnvironmentValidatingCache and EnvironmentAndValueValidatingCache, which are simplified interfaces to ValidatingCache which provide additional comparison of Environment contexts.
• Logging and tests.