Expand symbol! into type-level list instead of N-tuple

[soareschen]

Summary

Currently, the macro symbol!("hello") would be expanded into:

(Char<'h'>, Char<'e'>, Char<'l'>, Char<'l'>, Char<'o'>)

We should instead expand it into:

Product![Char<'h'>, Char<'e'>, Char<'l'>, Char<'l'>, Char<'o'>]

This is so that we can more easily define generic implementations work on the symbol types. For example, we can define a HasString trait that would be implemented for the type-level list of chars:

pub trait HasString {
  fn string() -> String;
}

impl HasString for Nil {
  fn string() -> String {
    String::new()
  }
}

impl<T, R, const C: char> HasString for Cons<Char<C>, R> 
where
  R: HasString
{
  fn string() -> String {
    let mut str = R::string();
    str.push(C);
    str
  }
}

[soareschen]

Closed by #36