add burn_private and burn_public transitions

[raghav-rama]

/ Burn / transition burn_public(public receiver: address, public amount_to_burn: u64) { return then finalize(receiver, amount_to_burn); }

finalize burn_public(public owner: address, amount_to_burn: u64) {
  let amount: u64 = Mapping::get_or_use(account, owner, 0u64);
  Mapping::set(account, owner, amount - amount_to_burn);
}

transition burn_private(token_to_burn: token, amount: u64) -> token {
  let difference: u64 = token_to_burn.amount - amount;
  return token {
    owner: token_to_burn.owner,
    amount: difference,
  };
}