Adding RenderContext trait so users can provide access to arbitrary structs without converting it all into a Value

[brosander]

I think that this will help users expose more extensive data structures to Tera without needing to recursively convert to a Value when rendering.

The existing api should still be compatible because both Context and any immutable borrow of a RenderContext will implement RenderContext so the render calls in tera.rs should still work with &Context.

I don't think I've changed any logic for the Context rendering. It is all confined to the RenderContext trait definition now though.

[Keats]

Nothing like that is going to be merged, all work is happening in https://github.com/Keats/tera2 which is a rewrite from scratch

[Keats]

#637 (comment) if you want to see details. If that still works on the new version I will have a look.
Ideally something like what ramhorns is doing would be great for everything that is not a hashmap tbh