Return bool from get_storage and set_storage

[chfast]

Continuation of #117

get_storage

For get_storage we can do similarly to get_code_hash and return false when an account does not exist, but this is very artificial case, because the get_storage is only used for the currently executed account. Any VM should not expect get_storage to fail.

set_storage

Similar story as for get_storage but worse. The set_storage returns evmc_storage_status. This one would need to be extended with EVMC_STORAGE_NO_SUCH_ACCOUNT.

[axic]

I think this still makes sense because the function still has an address parameter. What to do if a VM uses an another address? Not clear.

[chfast]

Well it would return a null hash in this case (as it does currently).

[axic]

Well it would return a null hash in this case (as it does currently).

This is not documented.

[chfast]

Is the documentation enough then?

[axic]

set_storage should have that NO_SUCH_ACCOUNT case.

I guess we can go with that for the moment, but if we do have NO_SUCH_ACCOUNT, we may just want to have the bool in get_storage and the design pattern is that we use bool where appropriate, but if the function needs more cases than true/false, then use an enum such as set_storage is doing.