Replaced match data participant* members w/ object

The match data object used to have separate members for participant a
and participant b. It looked like this:

participantA => {...}
participantB => {...}

Participant info structure is now:

participants = {
  'a' => {...},
  'b' => {...}
}

This change allows further modifications to the system that allow more
than 2 participants in a match.