Fix "random panic" in DecodeScalar

[the-eater]

Currently you can force a panic while decoding a Value wrapped in a container because all containers delegate decode to the default implementation, while Value only accepts calling decode directly,

This PR makes all containers call ::decode of their child type instead

code proof

use std::rc::Rc;
use knuffel::ast::Value;
use knuffel::span::Span;

fn main() {
    #[derive(Debug, knuffel::Decode)]
    #[knuffel(span_type = Span)]
    struct X {
        #[knuffel(property)]
        d: Rc<Value<Span>>,
    }

    #[derive(Debug, knuffel::Decode)]
    #[knuffel(span_type = Span)]
    struct Document {
        #[knuffel(child)]
        gamer: X,
    }

    let doc = "gamer d=(uwu)\":3\"";

    let parsed: Document = knuffel::parse("<inline>", doc).unwrap();
    println!("{:?}", parsed);
}

I would also like to note that the trait for DecodeScalar seems, confusing?

it's advised not to implement ::decode by hand, however there's no access to the type name in ::raw_decode, requiring weird workarounds to implement a polymorphic scalar while also following said advice

I e.g. currently want to support the following:

match "literal-match" 
match (re)"regex\smatch"

this either requires storing a MyScalarType enum in the context, or not following given advice lol