Merging blocks is lossy

I spent a whole bunch of time trying to figure out why individual blocks are fine, but after partitioning and trying to merge, many of them vanish. This breaks the LTN select boundary tool. `Perimeter::merge_all` appears lossless.

I found a workaround, but I want to fix properly. At least two of the problems:
- rotating a perimeter (part of `try_to_merge`) shouldn't have an effect, but sometimes, I _think_ it does. So we have a defensive copy in `try_to_merge` now
- some blocks are fine until we collapse deadends, then they break

Two of the areas I dug into:
- Leeds north, https://www.openstreetmap.org/node/2367939954, the inner hole probably is breaking it
- Bristol, https://www.openstreetmap.org/node/1401181776, the zorder is weird to start with