Cleos launches keos in slim branch

Conservative checktime fix for eos #2707

Cleanup coverage report.

Eager checks for the existence of accounts and permissions in actions

cleos get account pretty print, system newaccount

Changes to onerror and improvements to transaction_trace

Add python script for exchange tutorial.

get_account error reported by cleos

We do not need to expose payer information to the error handler.

Also we do not need to pass the original published time of the deferred transaction because now the `publication_time()` intrinsic will provide that information to the error handler.

Update eosio-s2wasm to generate new style call_indirect

* Avoid unnecessary check of expiration for generated transactions (instead just set the expiration similar to how we set the TaPoS). This also means that `test_transaction::send_transaction_expiring_late` can be removed.
* Avoid duplicate (redundant) validation of referenced accounts when executing a deferred transaction.
* Avoid duplicate (and erroneous) TaPoS validation when executing a deferred transaction.
* Avoid unnecessary validations of TaPoS, expiration, and referenced accounts for implicit transactions which can be trusted to be correctly constructed by the controller.

Also removed obsolete native handler for eosio::onerror. System contract wasm can still handle the onerror action for deferred transactions it sends (such as in eosio::postrecovery).

integrate slim net plugin

Remove nonce from system contracts; use nonce on nobody

This move of the submodule gives us an eosio-s2wasm that generates "new" call_indirect syntax. While cleos will accept either old or new call_indirect syntax, it's easier to interoperate with external tools by using the new syntax.