Add "ListAccounts" to cw20-base

[ethanfrey]

Allow a way to show all accounts. We can do that with the iterator, but it may use tons of gas if there is lots.

Maybe add some pagination here, like:

struct ListAllAccounts {
  // start from beginning if None, or right after this if set (use last address from previous result) 
  after: Option<HumanAddress>,
  // require some limit to how many on a page. Gas limits will cap it, we can enforce no more than 100 
  limit: u32,
}