Force defund instruction

[armaniferrante]

Currently, a "tombstone" is added to the first 8 bytes of an account when using #[account(close)]. However, this can leave an account in limbo if one closes it and a subsequent instruction immediately refunds it. This is particularly important for PDAs, which are associated with a user, since this effectively means the user can no longer use the contract with a previously used wallet.

To handle this case, we should consider adding an instruction to all Anchor programs that simply checks if a tombstone discriminant is present. If so, send out all lamports to a given, unauthorized destination address.

Related #604.