Allow arbitrary objects to be backends?

[sethaxen]

Currently the fallbacks in the package constrain the backend type to be an AbstractBackend. Is this necessary? e.g. as I think @oxinabox proposed on Slack, in relation to #11, it would be nice if a user-created ChainRules.RuleConfig could directly be used as a backend. Otherwise, one would probably end up implementing a duplicate object or implementing a loose wrapper around it.

How does this package intend to use these types in a way that can't be satisfied by overloading some interface functions?

[mohamed82008]

I thought some more about this and I think a wrapper around a RuleConfig makes more sense than allowing it to be a backend. In particular allowing RuleConfig to be a backend directly will mess with the HigherOrderBackend stuff.