Use consistent notification for the Yes fork and the No fork

[edmundedgar]

Clean up the inconsistency in naming with appropriate enums/constants to make sure we're clear which fork stands for "yes" and which for "no".

[edmundedgar]

Something like:

• Rename NewInstances.AddressPair's .one .two to .yesFork .noFork
• Define constants yesAnswer => bytes32(1) noAnswer => bytes32(0) for reality.eth answers
• Rename internal vars like child1, child2 to yesChild, noChild
• Leave children in ForkableStructure alone with indexes (don't bother making constants like yesChildIndex->0, noChildIndex->1) but rename internal variables child0, child1 to yesChild, noChild
• Leave forkResult which has 3 values (0 for not forked over a question, 1 for yesChild, 2 for noChild alone but rename internal child1 etc variables
• Everything on the L2 side is a reality.eth-style bytes32, but L2ChainInfo consumes forkResult as a uint8 then translates it. Make the forkQuestionResult internal mapping private, make getForkQuestionResult translate to a bytes32 and revert if no answer, and add an external isForkQuestionResultAvailable which returns true if there's an answer