WIP. Lifetime

[andy128k]

Related to #299

[Keats]

Thanks, I'll try it!
It is a bit worse in terms of ergonomics but if it works, it is a huuuge perf win compared to the current state.

[Keats]

I had a quick look in a flight yesterday but I don't think this will work. Rather than a reference, the functions need to have a lifetime bound other than `'static'. Afaik, the test passes because they share the scope

[Keats]

Made a branch with an example that would ideally work: https://github.com/Keats/tera/blob/lifetime-issue/examples/borrowed_trait_context/main.rs

I have to say I'm not even sure it's doable

[Keats]

I made a thread on users.rustlang: https://users.rust-lang.org/t/store-traits-with-same-lifetime-reference-bound/24437/2

[Keats]

In the end, Arc<RwLock<_>> does the job, no need for lifetimes