Replace all generic contexts with a single Cx type

[kjvalencik]

This allows users to take Cx as an argument instead of being generic over the Context trait. For example, this will allow us to implement TryIntoJs for closures.

This is technically a breaking change because several context types have been replaced by type aliases; if a user were implementing a trait over multiple of these types, it would now cause a conflict. In practice, users either didn't do this at all or implemented it generically over the Context trait.

We could keep the existing types, it just makes it harder to deprecate them in the future. For a Neon 2.0, I would want to eliminate the Context trait and only make it concrete. The special needs of function and module would become additional arguments.

[dherman]

I like this. I left a suggestion for a little documentation in the neon::context module docs.

The only thing I don't like is the collective baggage of all the different Context types. I wonder if it's worth making some of them doc(hidden)? Or if there's some way to structure the neon::context docs to make it easier to understand the minimal subset you actually need to learn?

[dherman]

We should add an example of writing helper functions on contexts in the module docs, right before the Memory Management section.

[dherman]

Does this mean if you already have an &mut ModuleContext you'd have to do something like &mut *cx to pass it to a function that takes Cx?