Implement first pass (ground)

[xurxodiz]

The algorithm makes two passes over the stage to fill it. This issue takes care of implementing the first, that includes plains, hills, gaps and pipes (empty).

Note that it doesn't mean we need to get the odds right; that's for another issue. This is just about making sure the functions are there and working.

At the moment of opening this issue, we can get vanilla gaps, vertical hill changes and flatlands.

[xurxodiz]

We might (most surely) scrap the two-pass idea entirely. But the functions produced here to place flatlands, gaps, etc are salvageable.

This issue will be closed now, but its spiritual successor is #17.