wasmtime: CacheConfig should be seperated into Config and Cache

[PureWhiteWu]

Now the CacheConfig is used both as the config and the real cache, this is not the ideal way.

I'd propose to refactor this and make this two seperate parts.

[alexcrichton]

It might be worth clarifying here that the caching system of Wasmtime hasn't really been touched much since it was first added to Wasmtime and isn't super well integrated with everything else. It predates most of the current API design so I think moving individual options to a Config like structure is likely a significant undertaking and I'm not sure that it's the best design to have.

In general I'm not sure how to configure the cache and ideally Wasmtime would be more flexible about its caching strategies to allow external implementations of caching rather than requiring a built-in one. Before refactoring the methods around Config here it may be better to see how features like #4155 will end up getting designed with Config or otherwise how they'll be configured.