Throw an error when `payable` is used in contracts

[platocrat]

Motivation

Viraz, in Optmism's discord server, asked, "what is the use "payable" in optimism?"
For developers, functions marked as payable will not work since msg.value is not yet implemented in the OVM.

Specification

OVM compiler should throw a warning to any contract functions marked as payable.

Backwards Compatibility

I see no issues with being backwards compatible.