Add derive macro for `template::context::Context`

[tyilo]

I would imagine that it would work like this:

#[derive(Context)]
struct MyContext {
    string: String,
    opt_string: Option<String>,
    list: Vec<String>,
    assoc: HashMap<String, String>,
}

Generated code:

impl Context for MyContext {
    fn visit<V: Visitor>(&self, visitor: V) -> V::Result {
        match visitor.var_name().as_str() {
            "string" => self.string.visit_with(visitor),
            "opt_string" => self.opt_string.visit_with(visitor),
            "list" => self.list.visit_with(visitor),
            "assoc" => self.assoc.visit_with(visitor),
            _ => visitor.visit_undefined(),
        }
    }
}

New VisitWith trait required for this:

pub trait VisitWith {
    fn visit_with<V: Visitor>(&self, visitor: V) -> V::Result;
}

impl<T: VisitWith> VisitWith for Option<T> {
    fn visit_with<V: Visitor>(&self, visitor: V) -> V::Result {
        match self {
            Some(v) => v.visit_with(visitor),
            None => visitor.visit_undefined(),
        }
    }
}

impl VisitWith for String {
    fn visit_with<V: Visitor>(&self, visitor: V) -> V::Result {
        visitor.visit_string(self)
    }
}

impl VisitWith for str {
    fn visit_with<V: Visitor>(&self, visitor: V) -> V::Result {
        visitor.visit_string(self)
    }
}

impl<T: Display> VisitWith for [T] { // Also for Vec<T>, [T; N]
    fn visit_with<V: Visitor>(&self, visitor: V) -> V::Result {
        visitor.visit_list().visit_items_and_finish(self)
    }
}

impl<K: Display, T: Display> VisitWith for HashMap<K, T> { // Also for BTreeMap
    fn visit_with<V: Visitor>(&self, visitor: V) -> V::Result {
        visitor.visit_assoc().visit_entries_and_finish(self)
    }
}