Change filters/functions/tests to borrow arguments

[mitsuhiko]

It's currently quite wasteful that a filter like lower always needs to convert into a String first. It would be a better interface if &Value or similar is passed, then a borrow to a &str out of that should be possible for the interface.

[mitsuhiko]

Entirely unclear how this can be done without GATs at moment or awful hacks I don't feel confident about.

[mitsuhiko]

I found a way to make this work now in #97 but unfortunately there does not appear to be a way to make this work for Rust versions below 1.61.0 without resorting to a lot of generated code and ugly trait bounds.

I'm not sure at the moment what a good strategy is here. I am somewhat willing to bump this up on minijinja as my own use of this crate does not require supporting old Rust compilers.

If someone has opinions, there is a poll now: #116