Updates and fixes to GAS_PAYER

[ggobugi27]

1. BUG: (read-msg) currently containsminer-keyset and total field. This should be removed.

2. Render useful info about the transaction as data that can be read using(read-msg) from GAS_PAYER

   • For regular pact commands: pact code
   • For cont commands: is-cont (or a bool that specifies if the tx is a cont), step, pact-id, pact code that will actually run

3. Charge gas for computation inside GAS_PAYER.

4. Implement a gas-payer-v2 or gas-payer-redeem interface to allow gas redemption once GAS_PAYER gets charged for computation