Support the `extcodehash` opcode

[xermicus]

Needs a supporting API method in the pallet.

[xermicus]

Assigned to @ermalkaleci

[xermicus]

To be more precise, check the comments in #77:

@ermalkaleci The pallet should be changed to write the zero hash if the account exists but has no code, and all zeroes if the account was not found. The returned status code can then either be removed (since the callee can check it by inspecting the output buffer after the call value) or changed to reflect what exactly was written accordingly.

[ermalkaleci]

@xermicus this is resolved

[xermicus]

Thank you!