Provide proc macro package for automatic IndexList<T> implementation on any index struct

[y-pakorn]

Currently, it would be kinda troublesome to use IndexedMap because we need to manually implement IndexList for every struct that we want to use as the index.

Rust's procedural macro feature enabled us to easily provide 1 line of code instead of manually typing the impl by ourselves for every struct we created.

Implementing this and reexport it in cw-storage-plus package (may be in an additional cfg features) will makes dev life much easier

Proposal: reduce this,

#[derive(Serialize, Deserialize, Clone, Debug, PartialEq, JsonSchema)]
pub struct Token {
  pub owner: Addr,
  pub ticker: String,
  pub identifier: u8, // <---- unique value
}

pub struct TokenIndexes<'a> {
  pub identifier: UniqueIndex<'a, U8Key, Token>,
}

// IndexList is just boilerplate code for fetching a struct's indexes
impl<'a> IndexList<Token> for TokenIndexes<'a> {
  fn get_indexes(&'_ self) -> Box<dyn Iterator<Item=&'_ dyn Index<Token>> + '_> {
    let v: Vec<&dyn Index<Token>> = vec![&self.identifier];
    Box::new(v.into_iter())
  }
}

to this

#[derive(Serialize, Deserialize, Clone, Debug, PartialEq, JsonSchema)]
pub struct Token {
  pub owner: Addr,
  pub ticker: String,
  pub identifier: u8, // <---- unique value
}

#[index_list(Token)]
pub struct TokenIndexes<'a> {
  pub identifier: UniqueIndex<'a, U8Key, Token>,
}