Make contexts composable

[ISibboI]

This crate soon provides a premade context, and if more are added in the future, the question arises of how a user would build a set of contexts. Ideally there would be another trait ComposableContext that allows to build a chain of contexts, such that each function/variable is first read from the last in the chain, if not found then the previous, and so on.

Writing should still only happen on the last context.