Added register_local_helper_rc to RenderContext.

[ExPixel]

Since register_local_helper and get_local_helper already make use of Rc in their signatures I figured there wouldn't be any harm in providing a way to create helpers using Rc directly without the reallocation required by Rc::from_box. It also allows using the same helper definition for multiple helpers with different names.

I also thought about fn register_local_helper<T: Into<Rc<dyn HelperDef + 'rc>>> but I think that might mess with type inference.

[sunng87]

Thank you for your patch! Instead of creating a new method for this, I'm +1 for your idea of using generics. I'm working on 4.0 so even a little break change is acceptable at the moment.

[ExPixel]

So I went ahead and just made it generic in a bit of a different way. This should still allow Rc<dyn HelperDef> (I had to implement that one manually) and Box<dyn HelperDef> but should also just allow passing closures or anything else that implements HelperDef. I can change render_helper's API and usages as well if this seems good.

[sunng87]

This looks great to me! Please go ahead.

[ExPixel]

Alright, so upon further inspection (and after more sleep) this doesn't really do what I set out for it to. The Rc::from(T) call is just going to produce an Rc<Rc<T>>. Without specialization there isn't really a way for me to implement things in a way that will allow T where T: HelperDef, Box<T> where T: HelperDef and Rc<T> where T: HelperDef to be used without extra allocations for Box<T> in Handlebars::register_helper and for Rc<T> in RenderContext::register_local_helper. Requiring Into<Rc<dyn HelperDef>> works but it messes up type inference and makes passing Box::new(new_helper) require some inconvenient type gymnastics. Going to have to mess around with trait bounds a bit more to get something that works decently well.