Make caches clearable and disposable

[Arithmomaniac]

Right now, there is no way to manually clear any of the caches (field, choice, MD5). In fact, since the caches share a global MemoryCache as a backing, they can't be cleared by garbage collection either.

• Caches should have a method on them to clear their contents.
• Caches should implement IDisposable, which does so
• Any provider that uses caches should have a ClearCaches method available on it to clear the caches.
• And provider that uses caches should be disposable, clearing its caches when done.

[KalinaTSD]

I'm going to present something here as proposal.