coordRange df::coord iterator for Maps module

[Bumber64]

Define a df::coord iterator coordRange for the Maps module, similar to Units module's citizensRange.

This should iterate all df::coords contained in a cuboid defined by two given df::coords. We might decide to not check if the input coords lie in the current loaded map bounds, because the function could be used to iterate block, region, etc., coords. (We can have a version that accepts a df::coord2d by setting z to 0 and returning the appropriate type.)

We can also accept a cuboid struct (#4595) and make use of it to simplify the iterator (rather than figure out which direction we need to iterate x/y/z.)