Add complete support for constant dynamic collections

[udoprog]

This adds supports for anonymous objects and tuples, like these:

const OBJECT = #{a: 42};
const TUPLE = (1, 2, 3);

I've also introduced runestick::Vec, which is a type-erased variant of a dynamic vector, essentially equivalent to Vec<Value>, but performs no conversion of its inner values and is capable of holding some associated methods useful for decoding inner values, like push_value and get_value:

let mut vec = runestick::Vec::new();

vec.push_value(42)?;
vec.push_value(true)?;

assert_eq!(Some(42), vec.get_value::<i64>(0)?);
assert_eq!(Some(true), vec.get_value::<bool>(1)?);
assert_eq!(None, vec.get_value::<bool>(2)?);