Review Host callbacks return types

[chfast]

• get_balance(), get_code_size(), get_code_hash().
  This should not return int to inform if an account exists. It would make it unnecessary complicated and a VM usually does not care (a contract might, but usually don't).

[axic]

get_balance(), get_code_size(), get_code_hash().
This should not return int to inform if an account exists. It would make it unnecessary complicated and a VM usually does not care (a contract might, but usually don't).

Based on the latest discussions it makes sense to keep these returning a boolean, just like get_block_hash is working, but with a slight change: if it returns false, it should not modify the supplied result pointer.

[axic]

The same should apply to get_storage/set_storage.

[chfast]

Handle set_storage and get_storage separately in #146.