Mechanisms placeable on halfslabs

Don't merge this yet. I don't know if this is okay.
`CanBeAt` is guaranteed to receive coords that are within the specified chunk. This means that you don't need to use UnboundedRel function, you can directly use `GetBlockMeta` for these coords. Of course, once you start moving from these coords later on, you will need to use UnboundedRel and check results, but now for this first one it's not needed.