lang: Close account macro

[armaniferrante]

We can add a macro to close account to recover rent exemption SOL. One example of this might look like

#[derive(Accounts)]
struct MyAccounts<'info> {
  #[account(close = sol_destination)]
  account: AccountInfo<'info>,
  sol_destination: AccountInfo<'info>,
}

Where the account discriminator is set to a special Closed flag, so that it cannot be safely deserialized again, and where the account's remaining balance get's sent to the sol_destination account.