WIP draft of conditionals concept

[colinleach]

This is an unfinished draft, NOT ready for formal review.

Suitable exercises to go with this might be vehicle-purchase or meltdown-mitigation, still to be decided.

As with #792, I'm putting unfinished drafts on GH while trying to get a feel for the scope of each concept.

There will be a couple more later today, but NOT the Arrays and Ranges concept. The current draft of that is getting very long, so I'll need to split it into multiple concepts: at least 2, maybe 3 (despite already excluding multidimensional arrays at this stage).