Custom key/context serializers

[rawilk]

PR makes it possible to define custom serializers for both setting keys and the context object. This allows for more flexibility on how setting keys are generated.

A new context serializer is also added, called DotNotationContextSerializer. This serializer maps each context argument into {key}:{value} pairs, and then implodes them together using :: as a separator instead of using serialize like the default serializer does. It will also detect a model class if it is keyed as model, and from that it will use the model's morph class intead of full name-spaced class in the key.

Before by default, all generated setting keys were hashed with md5, however that makes searching for multiple setting keys difficult. Now there is a new key generator, called ReadableKeyGenerator, which does not hash the key at all.