v0.3.4

• Update the string representation of Asset to include its type, i.e. instead of uusd:123456, the string representation is now native:uusd:123456
• Implement IntoIterator trait on AssetList. Now instead of asset_list.to_vec().iter() can directly do asset_list.into_iter()
• Implement Index trait on AssetList. Now can directly index into asset lists, e.g. asset_list[0]
• Implement method to cast AssetList to [astroport::asset::Asset; 2], which is used in astroport::pair::ExecuteMsg::ProvideLiquidity. Without this method, it is a bit cumbersome to parse asset list into fixed length slices.