Formalise expectations around corner identification

[PeterJCLaw]

Currently it is possible to have corners which are numbers that do not start at zero, yet this does not fully work. For Smallpeice 2023 we're going to work around this as we've (@prophile) numbered the corners 1 & 2.

Unfortunately we have quite a few places where we assume that the corners are zero-indexed numbers and that we can trivially map between the index of a team within the list of teams in a match and the corners identities. This also means that the display names of the corners are bound to their ids in a somewhat implicit manner.

[PeterJCLaw]

Note also that currently SRComp-HTTP appears to be using the "sort keys" functionality of JSON encoding, which sort-of helps here but also masks some potential issues.