onlyworlds 0.5.0

• Folder format v0.3.8, one id, one file (§4). A writer never leaves an element's id under both elements/ and the legacy spatial/, and a reader that finds one in both serves the elements/ copy. write_folder already wrote a whole folder under elements/ only, and Folder.all_elements() already served elements/ on a shared id, so nothing changes in what either does. FORMAT_VERSION (the format_version a written world.json declares) is now "0.3.8"; folders stamped "0.3.7" stay valid.
• Folder.in_both_layouts lists the ids found in both layouts, sorted: each is a folder some writer got wrong, which the spec says a reader SHOULD log.