* token-swap: Fix withdrawal all tokens slippage

When withdrawing, the slippage check is done before `min`ing the token a
and b amounts, which makes it possible to ignore the desired slippage,
and lose out on a lot more than expected.

This has an additional knock-on effect. When burning all of the pool
tokens, it becomes impossible to ever use it again.

* Check for slippage after getting the actual amount that would be
  traded
* Re-initialize the pool token amount on the next deposit if all pool
  tokens were burned

Fixes solana-labs#1629

* Fmt + clippy

* Deposit one side on 0 pool tokens just gives new supply back