Consistency in opcode casing.

[tigh-latte]

Currently through our code we have references to opcode and opCode, in both internal var names and error types.

This inconstency should be address before releasing v2, to prevent it become annoying and introducing the need for either accepting inconsistent behaviour, or releasing a breaking change.

I am personally going to argue for opcode and Opcode to be used, instead of opCode and OpCode, as in written English it is only one word and officially written as opcode.